I would think that a bigger bar would be better in almost all cases. Your kite will turn tighter with a longer bar and you don't have to turn the bar
as much in order to get the same turning capability as a small control bar. Especially in the case of a kite this size, a 55cm would be the one to
pick.
But that's just my personal opinion. I'd wait until a more qualified kitesurfing expert replies before you put all your trust into my post.