I think it will depend on what you're cutting, how tall and thick the grass is, and the terrain you're on, level, hilly, etc. For just one example: Bush Hog's web site shows the hp requirement for a 5' flat deck finish mower to be 15-35 hp. That's a pretty wide range. I used a 5' finish mower with a B2710 (about 20 PTO hp) and it never gave any indication at all of loading the engine. And I once talked to a dealer about 4' vs. 5' mowers behind a B7100 (13 PTO hp?) and he recommended the 4' one, but said he had some customers who were happy with 5' ones but that they were pretty heavy for the 3-point hitch to pick up without front end weights added.