Depends somewhat on what you cut, and how fast you want to drive. If the tractor's engine is bogging down, you'll have to slow down or only use 4 feet instead of the full 5' width... I use a 5' with 23 hp but I only cut in the winter, when it's dried out... only time I have to slow down is going up hill.
If you plan to be cutting thick grass or small trees, again, take smaller "bites" or go slower.