what size?

It's different for growing youngsters to developing players to settled adults.To keep life simple, for a growing young player the rough rule of thumb is that the top of the stick handle when the stick is stood next to the player should be around the top of the hip or just above. Please note that this covers junior players and stick lengths from 24" to 36" inclusive. Once they hit 36" / 36.5" the stick doesn't grow with you anymore - unless you want it to.

The 'standard' adult size of stick is 36.5" and the majority of adult players will use this size. However if you want to pick a longer length as that suits you more, i.e. you feel more comfortable with it or want the technical benefits it can give, then there's nothing to say you should not pick a longer length. Usually though a younger player would sit with a 36.5" for a while before making that call.