Don't have knowledge on Dakine bags, but if you're board is a 156 I'd suggest you get something longer than a 157. The extra length of the bag will be very beneficial to you when you travel overseas since you will be able to stuff all your gear,etc. into one bag. My friend has the Burton Wheelie GIg bag in the 166, and can stuff his board and all his gear (jacket, gloves, helmet, boots, etc.) in it and then load it up with all his other crap like clothes,etc.
I uderstand what you are saying, the thing is that the Dakine 165 wheelie bag is in actual fact loger then 165, its around 175cm.
So if the 157 is also longer the I am assuming its around 165. This is the reason I'm want to get an answer from someone that can measure the 157 bag to see its real size.