Many implement manufacturers on the web give tables that contain recommended minimum HP for their equipment (Maximum HP's too for drive shafts and gearboxes). I believe these recommended HP's tend to be conservative since buying an implement too big for a tractor (or breaking gears) would make for unhappy customers. However, these recommendations probably make good rules of thumb.
As Bird noted, soil conditions, tractor
ballast, tires etc. make a big difference in how big an implement a tractor can operate. The way an implement is used also makes a big difference.
A couple of days ago I found myself spinning all four turf tires trying to pull scarifiers through sod. I had to pull up the scraper a bit and then riped the sod in two passes.
I know the turfs limit the pull of my tractor, but I stick with them anyway. I have to drive on lawns sometimes. I can manage most traction problems by adding ballast, going slower, taking smaller bites etc.
Since I have turfs, I probably should aim low on the recommended HP's, but the scrapper calls for a higher recommend HP then my tractor has. I got a wide scraper because the next smaller size wouldn't quite cover my tire tracks. It does most jobs I have to do OK, and I can just plan on spending a little extra time doing the rest.