I think I read somewhere that when writing, you shouldn't describe characters by their characteristics
The tall man walked across the room
Is this true? Maybe I'm misremembering
This reads completely normal to me and I don't really see anything wrong with it.
If "tall" is what identifies the person in this scene it would be normal to use that if the narrator doesn't know the name of the person or wants to emphasize this person being "tall" for whatever reason, such as a small room or something like that.
Don't necessarily listen to any writing advice you find online as if it was the sole truth. Not everyone is an expert and if I write in my own blog that something shouldn't be done then maybe that is simply my opinion - not what everyone else ought to take as the holy grail. Read lots of opinions and you will come closer to the truth.
Maybe there was a reason this person said that you shouldn't use such characteristics. Or there is missing context. If the narrator knows the name of the character, why would they use such a description? Again, there can be reasons like I mentioned above, but if there is no such reasons you should probably use the name.
But without any more context about the kind of situations where you shouldn't do this such an advice is far too broad and generic. There are lots of advices like that for every kind of thing you shouldn't do in writing. It's the same in other fields, too. There are lots of advices about what a programmer should never, ever do for example - unless, of course, they know what they are doing and have a reason. Often this advice is for people new to the craft - many edge cases where things can go horribly wrong, so better stick to the simple stuff and an easy to remember guideline than to learn about every possible pitfall yet. There are cases where it makes sense to do something, but you should know when to do it and when not to do it.
That you shouldn't do something is alone not a good advice.
There needs to be context about why and when not to do something. Otherwise such advice is pretty much useless as it just cuts off a path for you without teaching you anything about the reasons.
Obviously you should take this advice with a grain of salt - I, too, am just a random guy on the internet posting his opinion about how not to do something ;)
Sometimes you'll see authors avoid constantly repeating character names by replacing them with descriptors. For instance (assume that all three descriptors are referring to John, the tall man who is Martha's son).
John walked to the window. The tall man looked across the field. Martha's son was feeling lonely this morning.
Don't do that. It's unnecessarily confusing. Character names disappear into the background, like other functional words. This reads like someone trying too hard --and making the reader try too hard. If you're wanting to tell the reader (for the first time) that John is tall and Martha's son, there are better ways to do it. If they already know, this doesn't add any benefit to them.
That, of course, assumes that we know the character's name. If we don't, that creates a challenge for the writer. We can try giving the mystery character a consistent descriptor (for instance, always referring to him as "the tall man") but that's a bit objectifying --especially if the descriptor refers to ethnicity or disability.
It is natural for people to notice any physical feature that stands out, but some words such as "tall", "big", "fat", "short" etc are not that descriptive. Adding "very" doesn't help.
If it is necessary to describe somebody's height, then it is better to use a comparison. "He was tall, nobody else in the crowd reached his chin."
Also, in terms of describing characters that will appear in multiple scenes, (like the main characters or supporting characters), it is generally better to not describe many physical characteristics at all. It slows down the story to spend time doing that, but more importantly, it increases the amount of information the reader has to memorize and remember.
This is why we say "Show, Don't Tell." Telling us a bunch of characters traits doesn't paint a picture in our mind, like beginning authors think it does. And these differences very seldom have anything to do with the plot. One or two might, if a girl is a natural redhead searching for her redheaded father, that detail about her hair is something the reader should learn early on.
But most physical attributes do not actually have any impact on the plot. The recommended way to introduce such characteristics is by having them make a difference in a scene, and perhaps even having them make a difference in a character's personality. A beautiful girl is treated as a beautiful girl, both by guys and other girls. Hilarious friends are funny and make us laugh. Tall friends are easy to find in a crowd, or can reach high shelves, or have to duck for doorways getting on the subway or getting into a car. You have to literally look up to them, and figuratively tall men get more respect in groups when they speak, than do short men. It's just a fact of human psychology.
If you describe your characters by the consequences of their features, you will create a visual scene and those are memorable to readers, those stick.
Typically as an author you should not burden the reader with facts about characters they feel they have to remember for the length of the story; if a feature is truly unusual and necessary then it should have some sort of consequences.
But I hasten to say that describing a scene or setting is NOT something readers feel like they have to memorize for more than the few minutes they need to get through it. Like real life, the sensory experience and seeing and hearing a setting is important, to ground the reader in reality. But like real life, the details can be forgotten quickly, and mostly what the reader remembers is what happened there, what was said and done and the emotional consequences of it.
So I am not saying do not describe things, but when it comes to characters (that are not one-time or one-scene walk-ons) it is best to portray them through actions and scenes and dialogue that suggests their physical and emotional characteristics. As the narrator, don't go on about how beautiful Cindy is; invent scenes and interactions and dialogue that show us how beautiful Cindy is. As a last resort, use a character to say how beautiful Cindy is. If she is truly beautiful, then people will notice, and act differently around her. Cindy will act differently because she has a lot of practice dealing with those reactions, unwanted advances, and compliments in inappropriate circumstances (e.g. getting hit on at a funeral).
Sometimes we tell, sometimes we show. The difference is that what we TELL has to be memorized by the reader, and what we SHOW in terms of a scene and imagined action and visualizations is far more easily recalled. Sight is our primary sense, and memorized "facts" not a tenth as powerful, and fading in a matter of minutes if only read once.
If something is not important for the reader to recall for more than a page, if it is just some detail to aid their imagination and help anchor them in a setting, then telling is fine. Otherwise, and in particular for characters and traits you want to be remembered, show is the way to go.