Size 5 and only size 5. When you improve yur ball control its not like your learning how to tap a small target(such as a size 1 ball), but your simply getting better at controling the mechanics of the ball as you familiarize with its physics. If you get used to a smaller ball you will start taking lighter touches on a bigger heavier ball and will not develop those muscles that allow you to dribble,drag ,roll the ball side to side.
The Brazilian soccer schools in the UK have, i think, proven futebol de salao balls (size 2) to be the most efftective out of the competitive match balls. You can obviously go smaller because the smaller the ball the harder it is to make contact with the sweet spot.
