Don't say:
I'm two years smaller than you. She's three years bigger than me.

Say:
I'm two years younger than you. She's three years older than me.

Note:
Great refers to the importance of a person or thing; Napoleon was a great man, Homer's Iliad is a great book Use great with words like distance, height length, depth. There is a great distance between the earth and the moon, informally, use great to mean something nice or good. We watched a great concert last night. If reference is to age, say young or old. Small and big usually refer to size He is big (or small) for his age.