First of all, you'll need more info than just the load capacity, you will also have to factor in the the diameter of the cylinder and the application. Are you concerned primarily with extension force or with retraction force also? The bigger your piston rod, the less retraction force you will get because there is less piston area available.
An easy to understand guide that I like to use is Fluid Power Data Book by Womack Educational Publications (214)357-0027.