One way to work definitions in from the viewpoint of a given character is to describe the reactions others have to them (or, if they are inclined to self consciousness, the reactions they think others have to them).
If someone is very tall, for instance, or has a large frame, people may "make room" for them, or crane their necks upward to look at them during conversations. If the viewpoint character is very short, they may be the one craning their neck upwards, and they may notice the kinds of reactions people have to petite people. If the person has unusual coloring or appearance for the place the story takes place, that may be reflected in the way people react to them too.
The shopkeeper grunted and said, "We don't get many Threskans here." It would have been nice to pass unremarked just once, but my pale skin and red hair always gives me away. At least she didn't take a broom and try to chase me out of her shop, like happened to one fellow I know.
If their coloring is similar to those around them, they probably won't get much in the way of reaction, but they can compare themselves to others they are describing if it makes sense for them to.
The shopkeeper had Amber-brown skin, a shade or so lighter than my own. Her curly, black hair cascaded across her shoulders in impractical abandon. I could never wear mine like that. Even cropped above shoulder length, it likes to go every which way.
Also, the way the person interacts with the environment can provide clues. Holding elbows or sucking one's gut in in to fit through tight spaces, standing on tip toes to reach something on a shelf, ducking down to avoid hitting their head etc.
I agree with others that you generally don't need or want to provide a laundry list of physical traits. Some readers do care about description, though. I know some readers who put books down if they don't get some sort of image in their mind of what people, including the protagonist looks like. I've read books that are so sterile of any description that my own image of their characters morphs in and out like they were some kind of doppelganger, and the setting feels like a swirling, gray mist with very few props.
I also think it's important that if you have characters who aren't our society's (unfortunate, imo) default assumption of a character (white, youngish, able bodied etc), it's important to get that information out as early as possible.
Some writers go overboard, though, and it used to be more the norm to write in a sort of "omniscient light" that felt mostly like limited third, in that a single character was the focal point of each scene, but the writer included details they likely wouldn't notice each and every time they interacted with someone (his ice-blue eyes narrowed as she pulled the lockpick from her golden curls).
I'd also forgotten how a particular writer I used to read a lot hated brown eyes and brown hair for her characters (the rare characters who actually had the most common combination in the world always ended up being bleached out by magic--bleh).
Don't feel like you have to make every character have startling and "rare" physical traits to make them interesting or memorable to readers, or to provide an excuse for description.