What size Basketball should I buy?

Question

I was going to buy a basketball so me and some friends can play at the park but I don't know the difference between the sizes eg. Size 7 or 5. Were all 15yrs, but there are boys & girls playing, any ideas?

Thanks x

Answer 1

Go with size 7 since you said there was going to be boys, is not that large either  

Answer 2

one that  has it signed by leBron james

Answer 3

7