Will 20lbs dumbbells get my biceps bigger?

Weight doesn't matter it's all about the reps you do. If 20lbs feels like a good weight use it, do a workout with about 10 or 11 reps x3 see if you can "Feel the burn" in your biceps. If you can then great keep doing it if it seems a little light do more reps same amount of sets. if it's to heavy take it down a notch do about 7 or 8 reps same amount of sets. Least that's how i was Taught.