Indeed muscle size is increased most by 8-12 rep sets(Aim for 10, to failure of course.[Spotter!]) according to the encyclopedia of bodybuilding. Not an exact quote but thats the range for muscle size building.
*Also, try to work opposing muscle groups in each workout like chest/back or triceps/biceps, it will help to maintain muscle balance: try one set for biceps then one for triceps, then bi, then tri, so one gets a short rest while the other is worked.
*The first set only does a little, the last set, when the muscles are fatigued, stimulates the most muscle growth and strength.
*Don't crank out the reps, the best rep is 4 seconds of work 2 seconds for return, slow in other words. Most lifters only do 2second 1s which is ok but not great, a few just give the old heave ho and throw the weight up, which does little, no matter what your goal(strength, mass, definition, etc.)
These are well studied sports medicine facts, backed by more than one source, not just some thumb and eye estimate.