What size mouse should i feed my snake?

i have a colombian boa, he is about 2 and a half feet or so. and what i have been feeding him is frozen and thawed hopper mice, and they seemed kinda small for him, they were smaller than the thickest part of his body, and i kinda want to start feeding him the adult frozen thawed mice, they are probably about the thickness of the thickest part of his body, maybe a little bigger, what do you think it would be better to feed him?