Aside from being a little harder to turn, you'll have just a little bit more power, but it's not enough to cause a problem, you just might need a little more depower. As the wind picks up it's a little harder to hold an edge, which can be solved by using a smaller kite. Smaller boards have the opposite problem.

For a freestyle board you might want to go slightly bigger as long as it's the right shape, as you'll be able to load up more power into it and get more pop.

The width and shape are very important and makes the board feel totally different, for example I currently ride a 133x41 liquid force nova, at 65kg and played with my friend's 138x40 (with straps on it) and a slightly different shape and it didn't feel that much bigger. But when I put straps on my 137x42 (the board I usually ride boots with) it felt like a boat.

So it really depends on the shape of the board, the rocker, etc.... you really just have to try it unless someone who currently rides it can just tell you what size to look for at your weight