It's the prop size that's critical because it's the prop that creates the thrust. There are formulae available but they're quite complex involving motor losses, propeller efficiency etc so to get a good estimate of the thrust available most of us simply use one of the many motor calculator programs available, some online some not, some free, some not. See
http://www.rcgroups.com/forums/showthread.php?t=606703 for a comprehensive listing of them.
Steve