Replying to Powder ski size help please!

ok so usually I'm fine deciding on stuff like this, but I'm a bit stuck. i want to get the k2 shreditor 112 for an all mountain/pow ski. I'm 5'10/5'11 and i currently have some 176 candide 1.0s for the park. i don't know whether the 179 shreditor is a bit small but then maybe the 189 a bit big?
i want a playful ski for spinning of natural stuff and pow. i would go big but i don't want them to seem really heavy and slow.
thanks for any opinions!