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When describing the physical features of my more important characters, I often don't add much. I of course describe important features, especially if they will be relevant in the story. For example Harry Potter's scar. Part of my problem is my mental model of my characters is their personality and not their physical features.

As my readers, what is the bare minimum you need to know about the main character(s) physical appearance? Can I get by casually mentioning their height and gender and, if needed, their symbolic plot driving scar? Do you need more? If so what? If length is important, I am mostly interested in answers that focus on long short stories or novella length works. I would be curious to hear about other lengths as well if you have insight on that as well.

Describe the important bits

In my opinion Harry Potter is actually an excellent example of how to describe characters. Harry is described in detail because his exact appearance has important bearing on the story. Ron's appearance is slightly less detailed but shows how he is clearly related to the other Weasley's. Hermione is describe as bushy haired with buck-teeth, nothing more is known about her because it isn't important. Rowling has said Hermione could have been black but since it didn't have a bearing on the story it was never included.

Don't add things later on

One thing you need to avoid in describing characters is changing that description later on. Within the first few chapters your readers will have developed an image of that character in their mind. This will be based on the details you gave and biased by their own experiences. If you later add details to the character that conflict with this vision they will reject is and may lose interest in the book entirely. This is known as 'breaking the contract with the reader' and should be avoided.

Readers need to make an image of the character in their minds to imagine the story, as unlike in TV shows and movies, they can't see them.

Try to describe everything that makes them unique, or attractive, other than their height. Is their skin colored? Are their hair curly or probably jet black and straight? Are their eyes a beautiful shade of green? Do they have freckles? Is their nose too crooked? Is their body too much broad or muscular?

The main hint here is to point out the things someone would notice in them the first time they would see them. You can obviously omit the features that are common among other people belonging to your story. Like, if it is US/UK based, you don't have to tell if a person is white skinned. If it is Asia based, you don't have to mention if the hair is dark or black. Similarly, you only have to mention the height if it's longer or shorter than the usual.

I'd agree with @linksassin here. DO NOT add the details anywhere other than the first few paragraphs where the character is introduced. The reader might then make up a total different image of that character before your explanation, and it would be very difficult for him to change that.

I think the simple rule is: Describe what is relevant to the story. Don't mention anything that isn't relevant to the story, as extraneous description just bogs things down. Mention things when an observer would see them.

For example, if in chapter 10 it is vitally important to the story that Fred is very short and thin -- maybe it's crucial that he can fit through a small passageway that no one else can, whatever -- don't wait until chapter 10 to mention this. Anyone seeing him would presumably notice this right away, so it should be one of the first things you tell us about him. If you never mention it, the reader might get a picture in his mind of Fred as someone tall and fat, then when you tell us he's short and thin, suddenly we have to re-create our mental picture of the entire story up to this point.

If something wouldn't be apparent to others, you should usually wait to say it until another character notices it. Like if Fred has a mole behind his right ear -- I can't think of why that would be important to a story, but whatever -- that would likely not be the first thing a person would notice about him. Wait until, say, his girlfriend his kissing his ear and have her say, "Huh, you have a funny mole behind your ear" or some such.

In general, if you don't say, most readers will assume "normal". If you don't say how tall Fred is, readers will generally assume he's about average height. If you don't say what he's wearing, readers will generally assume something normal for the setting. Like if you say he's a lawyer arguing in a 21st century American court, readers will expect he's wearing a nice, conservative suit unless you say otherwise. Again, avoid jarring surprises. Don't tell us he's a lawyer arguing before the Supreme Court, go on for 20 pages about the court proceedings without mentioning his clothes, and then suddenly tell us that he's wearing bermuda shorts, a garish T-shirt, and sandals. That forces the reader to re-visualize the whole scene, and breaks immersion in the story.

But avoid irrelevant details. If it doesn't matter what color shirt he's wearing, don't bother to bring it up. That kind of thing just distracts from the story. Like you wouldn't write -- or I hope you wouldn't write, "As Agent Jones entered the parking garage, three assassins jumped out of the shadows and attacked him. The first assassin was wearing blue dress pants and a yellow shirt. His shirt had a pocket on the front and white buttons. He had a digital watch on his left wrist that was displaying the correct time. The second assassin had a slight stubble indicated he hadn't shaved that morning ..." etc. Unless such details prove to be important, this would totally ruin the excitement of the scene.

Perhaps I should say that when I say "what is relevant", there are many ways in which details could be relevant. The most obvious is if an event in the plot hinges on something about the character. Details can also be important if it helps the reader to understand your character. Like it might be very relevant to say that he wore blue jeans and a T-shirt to his sister's wedding, even if this doesn't directly affect anything he says or does at the wedding, because it tells us something about the man. Depending on other things you tell us, that might mean that he is a very informal person, or that he doesn't care about social conventions, etc.

I am much like you in my process in that I see my characters’ persona but not their appearance unless it is important.

I mention my MC’s height, his eye colour as that has bearing later on and that some regard him as handsome. I did a google search to discover that the eye colour I had chosen restricted his hair colour to three possibilities - I chose one. Later, a cousin of mine asked me to describe this character as he has a much more visual approach.

I have characters that are only described by something that makes them unique. I have one who wears very expensive suits and drinks only the best scotch. It is up to the reader to take him and colour him as they please. Is he short, tall or average height? I know what I think he looks like, but his words and actions are what matters. I give him a name and the reader can run with that.

Others, where it matters, I describe in broad strokes. Visualizing characters is something we all do.

Detailed descriptions can be great, painting a clear image of a character.

The freedom to make a character look how the reader pleases is something I enjoy. It allows the reader to invest those traits they imagine the character possesses based on hints given by the author.

It depends on who's doing the describing.

Some objective, omniscient narrator? Describe features objectively. Focus especially on anything that's relevant to the story (e.g., a physical attribute that becomes a plot point later on), anything that reveals their role in the story (e.g., a uniform), anything that shapes the character's self identity (e.g., insecure because of a lazy eye), or anything that affects other characters' perceptions of them (e.g., a slumped posture).

But if your narrator is another character (or if you're using a close third-person where you're in a character's head), describe the details the narrator would notice. This gives insight both into who the new character is and into the narrator's world view. Maybe an assassin sizes up everyone she meets in terms of whether they might present a threat. A womanizer describing a woman is going to focus on aspects of her physical appeal and take educated guesses about her psychological vulnerabilities. A high school teacher might not pay much attention to whether a student is a sophomore or a junior, but the students likely would apply these labels to each other.

In general, use some very specific detail that makes each character different from all the others. The reader can latch onto that as a label to remember who's who and fill in a lot of the irrelevant details. These labels should be the features that dominate our impressions of these characters.

Traits don't just have to be visual. Smells, sounds, and textures can also be interesting features of your characters. As can mannerisms, tics, accent, posture, etc.

Do not assume some standard lump of human and then describe only the attributes that make them different from that standard, especially if that standard is yourself. For example, if you mention the skin color only of black people or the gender of only the female characters or the age of only the elderly ones, you risk coming across racist, sexist, and ageist. (Of course, if you're in the head of a racist, sexist, ageist character, then it might be appropriate.)

Robert Sawyer is well known for including all these details for all his characters, in order to create a diverse, interesting, and fairly treated cast. The readers aren't left to assume the skin color of the new character is like their own.

At the other extreme, you could never mention these details for anybody. But then you can't write a story about race or gender or age, or even one where those even factor in. It might also seem a bit generic to readers. People do notice skin color, gender, and age. If your story doesn't, it might come across as rather bland.

A character's race, sex, and age should not be the defining trait that makes them stand out from the rest of the cast unless it's central to the story: the one woman in the boardroom, the one Hispanic left in the gentrified neighborhood, the one employee with decades of experience in the new startup, the boy in the wheelchair who takes the elevator alone while his buddies race down the stairs.