Don't just go with waist width when looking at a potential board. What really counts is the width at the inserts (varies with sidecut!) you would be using, and also the stance.
Anyways, with an 11.5 Ambush you should be fine on most of the boards with a waist from around 25.3 cm. Depends on the board of course but that's a general marker, at least from my point of view.
Alright thanks for the Input. The board i have been lookin at has different sidecuts; 8.3 and 7.5; think that will be kosher?