here's a adverbs hover hint sheet that might be of use to some ...
Why is "Her family is very big" not correct?
It would only be "your family is very big" if the sua was capitalized (es. La Sua famiglia). Of coarse if your talking to someone, you cant see if "sua" is capitalized or not so you would have to go based on context. If it's written down tho, the capitalization of the "S" in "sua" will determine if it's meaning is "his/her" or "your". Vostra/o can be used too, although its old-fashioned.
Just like Voi (old-fashioned) vs. Lei (used alot more often nowadays) to mean "You" (formal)
Of coarse these words are for talking in a formal setting. Otherwise Tu/Tua/Tuo/Tuoi are perfectly fine for meaning You/Yours.
Your family is very big. " la tua famiglia e' molto grande" or " La vostra famiglia e' molto grande"
As far as I know, "grande" really means size and not length. "Alto" would work a lot better