I buy whatever I like or make my own board. Size doesn't really tell you much. I have a liquid force concept Mauricio Abreu 135, and a Jaime 132, they look to the eyeball almost identical, but the concept is super hard, no flex, it beats up your knees, also it has an exteme amount of rocker so it's like you are driving a dump truck, and the amount of rocker acts the opposite of how you would expect, you think it would give you more pop but it gives you less pop because it's impossible to get decent speed. The Jaime by contrast has just the right amount of flex not so much that it bends when you are cruising and slows you down, but enough that it doesn't wreck your knees, it has just enough rocker, and builds up speed easily, drives upwind easily, then after it starts planing fast you can lean just on a tiny bit of the edge and really get cranking so you can build up a ton of speed an pop hard (I'm running small fins on it). The only time I use the concept is if it is blowing 50mph and want to plow through the water like a bulldozer for control. But to the eye they look identical. I love the jaime so much, my favorite board, no affiliation to north, I don't ride any of their kites, but I really like this board. I've demoed a bunch of other boards, every one is different, the specs on width and length don't tell you that much.
Yeah I've got the 128 x 38 jaime and it's a world of difference from my recoil 133 x 40 (which i've since sold). Both are 09 boards and what's surprising is that I can go in as light of wind with the jaime. The diff in the flex of those boards is amazing... the recoil was very stiff compared to the jaime. I'd like to get a smaller jaime if they made them.
Someone told me if the shape of the board and rails is right then the flex and fin size doesn't really matter. So who the hell knows what's up or down