Does a physical description have to be specific to make a character feel real, or can the physical description be general and the details be left to the reader's imagination?
A character does not have to be described at all to feel real. In many stories we are told little of their appearance beyond whether they are male or female, and occasionally not even that.
Where physical appearance is described it can really go no further than to place the character in a general class of people (An cavalry officer. A southern belle. A poor sharecropper. Etc.) and specify certain significant physical traits (fat, thin, bald, etc.).
You can go into more detail, but language does not give us the facility to easily describe the particular details of face and form that allow us to recognize individuals in real life. If a character has a face to a reader, it is the reader, not the writer who has supplied it, at least until the movie gets made, at which point the actor provides the face.
What makes a character feel real, more than anything else, is their behavior. When the characters in as story do not feel real, it is because the author has treated them as props instead of people, has made them behave in a way that advances the plot or make the point the author wants to make, but is simply not the way a real person, particular not that real person, would behave.
I certainly agree with Mark. Not much to add. But always remember POV. Who's telling the story?
Without a doubt Emma Richards was the most beautiful girl in the world.
A valid opinion if your narrator is a love-struck high-school boy.
And, for me, "relative" description are far better than specific facts.
Mr. Roberts was 6'2" but his wife was only 5'4
That is a rather bland statement of fact.
"You don't want to be late on your first day," said Mrs Roberts, wrapping the scarf around her husband's neck and standing on tip-toes to kiss his cheek.
This conveys the required information without pausing the story.
To Mark's useful answer, I add: On occasion, a character's physical appearance is relevant. Others react to us in ways that depend on our appearance: size, age, distinctive dress, etc. Also, the kinds of things a character would do, or not do, may depend on appearance. This ties in with behavior.
For example, in one of Raymond Chandler's novels (featuring private detective Marlowe, narrating in the first person), Marlowe begins by telling us that he was well-dressed in a dark blue suit, with shirt and tie, and was sober. Anyone familiar with the series would know that was very unusual for Marlowe. He then goes on to tell us why he was dressed, for this occasion. It makes for very entertaining reading. We also know (from other books in the series) that Marlowe is physcally very large, and this affects how others deal with him.
Marlowe often describes the others he meets, then tells us his first impression, based on appearance. He infers the person's internals from the externals that the person chooses to present. Similarly, in many Sherlock Holmes stories, Holmes infers a lot about a visitor, merely from appearance (thanks to stereotypical dress, in that era).
But again: Those things are relevant to the story. Otherwise, I suggest you take Mark's advice, and omit physical details that are irrelevant. Focus on behavior.
You have to ask yourself, "does it matter in any specific way?".
The answer is usually "no."
I usually describe characters very generally, and NEVER in prose. If they are described, they are described by characters or by themselves, in dialogue, and that dialogue is always connected to the immediate action of what is going on. We describe the characters because it matters in some way to that immediate action or to the larger plot. Examples:
"If I were as tall as you with forty pounds more muscle, sure, but I'm not. You do it."
"We have to go see Sherry. Now, she's four foot five and ninety pounds, looks like a kid from middle school, but don't let that fool you. She's got the power and she wants to make sure you know it, you know what I mean? Show some respect."
Physical characteristics can matter to the plot. An attractive character may find it easier to seduce somebody, an unattractive character may find it harder. It can be plausible a young college girl meets a very handsome young man, a stock market worker dressed in a suit, that offers her a ride somewhere, or she agrees to go get a drink with him. It is less plausible if our young college girl meets a mismatch, a fifty year old guy, 75 pounds overweight, wearing a tee shirt and loose pants, sporting full facial hair. She is not going to agree to go get a drink with this guy, or accept a ride.
Likewise, a very tall person can reach things a very short person cannot, a guy that looks like the high school sports star is more likely to handle himself in a fight than the guy that looks like he gets frequently bullied.
If you need to be specific about description, do not do that to try and fix an image in the reader's mind.
Do it because the description has specific effects on the character's decisions, thinking and actions that will influence the course of the story. If I ever describe a woman as being particularly small breasted, it will be because this can influence her psychology and self-image of her own attractiveness, and perhaps the perceptions of others about her attractiveness, and those are going to influence her decisions and thus the plot. You have to ask yourself does it matter, in any specific way? The answer is usually "no."
Secondly, avoid "telling" instead of "showing". Written in the prose, by the narrator, can work if it is kept to one sentence, the first impression kinds of things your MC might see as a first impression. Stay general, do not describe "average". That is the base assumption of the reader. Only describe noticeably unusual features that are necessary for the plot to progress later.
Thirdly, avoid hyperbolic description. A girl does not have to be extraordinarily beautiful for the hero to fall in love with her; unless he (or she) is shallow as hell, and then that isn't love anyway, it is just lust.
You don't need the perfectly formed, and it is best for the story if your characters are not paragons of ideal femininity or masculinity, are not over-the-top homosexuals or even villains. They should have flaws and weaknesses. I give every significant character (meaning, they appear in multiple chapters) some exceptionally good trait, and some particularly poor trait, and sometimes to help make them distinct, some weird but harmless quirk or habit, in speech or behavior or dress; a marker for the reader to connect them. "Oh the hat guy..." [The only character always wearing a hat].
Give as much physical description as is relevant to your story.
Don't bog down the story with irrelevant details.
As Agent Jones entered the room, he was suddenly attacked by an assassin! The assassin was wearing blue jeans and a green shirt. The shirt had six buttons down the front. The blue jeans had a slight tear behind the left knee. He held the jeans up with a brown belt ..."
Well, I presume you get the idea. Such a description would ruin the drama of the scene. It probably makes no difference to the story what the assassin was wearing or what color hair he had or how big his nose is or any of thousands of other details of his appearance. The reader will picture something in his mind. Just leave it at that.
Of course you should describe a character when it does matter. If you're writing a story set in Alabama in 1850, whether a character is black or white is very likely to be relevant. If you picture a character as big and strong and physically intimidating, you might want to mention this so that the reader understands why others take notice when he enters a room, or why people back down from confrontations with him. Etc.
You rarely need to go into great detail. "Jack wore an expensive business suit" might be all you need to say. Whether the reader pictures him wearing blue pin-stripe or black or orange probably doesn't matter.
"Sally was the most beautiful girl he had ever seen" might be as much as you want to say. If you start describing her hair style and body shape and so forth, your idea of "most beautiful" might be different from the reader's, and so you could detract from the picture rather than adding to it.
If some little detail does matter, then yes, bring it up. If the hero is confused for someone else because they were both wearing green shirts, you probably want to mention the color of each of their shirts somewhere. Etc.
I am not generalizing anything but most of the time it is the story and the characteristics of the character which are of significance. Not much attention goes into the looks or the physical appearance unless explicitly made to realize.