"you" has 3 meanings in French:
"tu" is singular and familiar, what you use with family, friends, colleagues. Masculine: tu es petit; feminine: tu es petite
"vous" is singular and polite/respectful when you talk to people you don't know or policemen, or judges or anyone with authority. Masculine: vous êtes petit; feminine: vous êtes petite
"vous" is also the plural "you all" when you address a group of people. Masculine: vous êtes petits (all males or a mix of males and females); feminine: vous êtes petites (all females).
Therefore, for the English "you are small", depending on context, you can use 6 different translations.
It is true that many say "you all" in English, but it is a colloquial form and not the best example of American English. We commonly say "you" whether referring to a single person or a group of people. By showing "vous" as "you all" suggests that this is the expected use; rather it is simply "you" (plural).