It depends on the 2 year old and how there body is built. But my 2 year old nephew wears a size 3 diaper.
I've also had a nephew who's now 5 wear a size 6 diaper when he was 2 years old because he was a little chubbier and taller than the other 2 year old nephew.
Most average sized 2 year old toddlers wear a size 2 or 3 diaper though.