"You are very small."
Translation:तुम बहुत छोटे हो।
You can also write 'तू बहुत छोटा है।' but the difference between the two sentences is not that one is singular and the other is plural though one uses छोटा and the other छोटे.
The तू form is the most informal to be used for very close friends and family and the तुम form is used for anyone of age with you or younger. Both may be referring to just one person.