Which size?
The choice of a ball is made based on weight.
Kids ages 6-9 will need a ball weighing between 200 and 220 g.
From ages 10-14, the weight should fall between 230-250 g.
Ages 15 and up should use an official-weight ball between 260 and 280 g.
You can train with a Size 4 ball, but in competition, you'll use a Size 5 (official size) ball.