The thing I have heard repeated, but don't know if it is anything better than a recommendation, is 3 to 5 PTO hp per 12" of cutting deck. PTO hp is typically several hp lower than engine hp.
Check out your PTO hp and see what that is. Then consider the lawn conditions.
The way it was explained to me regarding the 3 to 5 range is that in some areas of the country where there are arrid conditions, a larger deck can be used, consequently lower hp requirements, while in other areas lawns can grow pretty lush and pretty tall, consequently higher hp requirements.
I know I have bogged down my 24hp tractor (~19pto hp) with a 60" deck a couple times too often -largely because I don't like mowing and my lawn gets pretty thick in some spots. I usually know where I am going to get in trouble even before I pull out of the garage.