Depends on how much time you have to cut it. Any ZTR over 52" would be fine but a larger deck is going to get it done faster. Any commercial grade equipment is going to be fine also so I am not even going to get into manufacturers.
Here are some ideas for HP to deck size based on what most MFG's use. If it's commercial grade they already figured it out so dont worry about it.
19HP for 48" deck is ok
23HP for 52" deck is nice powerful combo.
25HP for 60"
27-30+ for 72" decks