If you intend to move any wet snow, or lots of soil, then you may want to have that blade set at 30 or 45 degrees (about the vertical pivot). Measure your rear track, from outside sidewall to outside sidewall. Take this measurement and divide by .866 for 30 degrees or divide by .707 for 45 degrees. This will tell you what size you need to cover your wheel tracks set at that angle.
eg: rear track = 47" set at angle = 45 deg
47/.707 = 66.5" So at least a 5.5 foot, more likelya 6 foot would do.
Depending on how much weight you are trying to move, and how much traction you have, you may want to go shorter. Or you may want to add rear chains or ballast (or both) and go longer. Your call.