Is it grammatically plural? In "You are tall", "you" can be singular, and then so should "are" be, if I am not mistaken.

(I'm not a native English speaker)

“You” was exclusively plural (singular equivalent was “thou”), but its meaning shifted to be either singular or plural a few hundred years ago. So “are” was indeed a plural-only conjugation until that shift happened.

Now “you” is shifting again: in formal English it is still singular or plural, but in colloquial American English — at least where I’m from — it can only be singular. The plural form is “you guys” or “y’all”, depending on dialect. (The “guys” in this pronoun shouldn’t be confused with the noun “guys”, which is usually only used for boys and men; “you guys” is used to refer to groups of any gender).

Exactly. There's loads of examples of such shifts in other languages, for example in Portuguese the formal you is conjugated in grammatical third person despite being semantically second person, arising from people being formally addressed in the third person ("Does the Right Honourable gentleman agree that...").

Same is true in Spanish, where "usted" ultimately derives from "vuestra merced".

For a completely different example, "on" in formal French means something like "one" in English: a pronoun for a general, unspecified person; as such, it takes singular conjugations. However, in colloquial speech, it means "we" and has almost completely replaced "nous" as a subject pronoun.

Thus: "il est" (he is); "on est" (we are, colloquial language); "nous sommes" (we are, standard language)