It depends on the length of the blades. It's my understanding, you start with a maximum speed you want for the outside edge of the blade, and use that to figure out the rpm of spindle, then figure out relative pulley sizes so you get that spindle rpm with the speed you want to run your engine.
Usually measured by tip speed. Meaning the speed, FPM or Feet Per Minute, at which the tip of the blade moves.
Use your favorite search engine and type: "tip speed mytractorforum.com", w/o the quotes, then hit enter.
It should come up with a few posts on MTF about tip speed and formulas to figure it out. And formulas for changing sheave sizes to get to where you want to be.
Also posts on figuring out tip speed for impellers on snow blowers.
There are limits on tip speed, don't go too high of a tip speed or things start to fly apart.