Im 5'10 165 and ride my 157 banana magic for pretty much the same thing as you, i hit park jumps and natural hits as well. And my pow board is a 158 charlie slasher.
157 seems about right for all-mtn style riding for me. But i just got a 151 for learning to jib and just farting around, it isn't as good for going fast though.
Dude, suckin at somethin' is just the first step to bein' sorta good at somethin'