Max air pressure should be listed on the tires although you may not wish to put maximum pressure in the tires especially on the rear. I find that rear tires pretty much work for all sizes with around 12 PSI. You want just a bit of squat/giveto the tire when you have the heaviest piece of equipment on it. This gives the smoothest ride and best traction.
Fronts take a lot more especially if you are running a FEL. For my fronts with FEL, I usually run about 10 PSI below max pressure. That allows me to carry max load without fear of running the tire off the rim.