First off, political correctness isn’t really a thing here (and honestly, that can be a good thing). It’s totally normal, and not considered offensive, to refer to people by their appearance. For example, an Asian person might be called “el chino” (“the Chinese”), a Black person might be referred to as “el negro” (“the Black”), and if you’re older you might hear “viejo/vieja” (“old man/old woman”). The same goes for body types: someone heavier could be “el gordo” (“the fat one”), while someone thin might be “flaco” (“skinny”).
These nicknames are usually seen as normal and even affectionate. So if someone calls you “negro” or comments on how you look, don’t take it as an insult, 95% of the time, they’re just being friendly. The only real taboos are badmouthing soccer (football) or insisting that the Falkland Islands are British. That’s when you might actually get yourself in trouble. Hope that clears things up!