One easy way to do this is to go to the Electrifly motor configuration page.

Click here: [link=http://www.electrifly.com/config/]Click me![/link]

For a 7.2 volt configuration you can go either inrunner or outrunner ( try both to see, though the inrunner will require a gearbox ).

Try a 12in prop, ( you can also try with a 14" prop just to see what the calculation comes up with ) and add an ounce or two to your plane's weight to be safe. Do figure out what your max prop size can be by putting the plane level with the ground and seeing how much clearance you have from the prop line to the ground, then subtract at least 2" for a good estimate.

Select "Sport" instead of "trainer" for the plane type so you are assured adecuate power, as you can ALWAYS throttle back.

I find that the recommendations made are pretty good and have worked very well for me.