You have 20 horsepower at the power take off. The usual standard is 5 HP per foot. So that works out as 4 feet for you. However....ahem.....if your doing lighter cutting, willing to go slower, willing to maybe keep your blades sharper, willing to mow a bit higher, willing to take a partial cut in thicker stuff you can get a lighter duty 5 foot one to work for you. You'll need weights on the front of your tractor EVEN if your mostly level. For real. Get front weights. A Landpride 1860 is a nice choice as its really 4.75 feet wide and comes in green!