One of the options with my lawnmower are different size decks to suit whatever you want to cut. They start at 48", 60 is the common one, 72" and 84" is the biggest. Mine has 23.5 horsepower, and the newer models go up to around 28 HP.
I would think 45 to 50 horsepower would be enough, depending on how fast you wanted to cut and how infrequent you did it.
David from jax
Lawnmower is Steiner Diesel, by the way.
David