The easiest thing to do it probably trial and error. What are you currently running?

For example - not really a good 3D example but an example none the less;

I have a Magnum .91 on a Spacewalker II - with a 14x8 I get great snap rolls and great all around performance. However with a 16x4W I don't get snaprolls that are as tight (by spacewalker standards) but the airplane will hang on the prop for a good long time which it won't do with the 14x8. It'll also tractor straight up at a very constant speed until I'm ready to stop. With a 14x8 I still get great vertical but the speed is much more than with the 16x4.