From what I learned while shopping around, some of the manufacturer's sites show the recommended minimum PTO horsepower for their respective shredders. I wanted to use the medium duty Bush Hog 286, and BH's site showed it needs at least 40 PTO h.p. so I had to step up the size of my tractor purchase to accomodate it. On the other hand, a light or standard duty hog requires much less tractor.
I realize that more factors come into play than just the PTO (e.g. carrying capacity of the 3-point system), but maybe that will get you headed in the right direction.