There are 2 things to consider, PTO HP and weight. If you have flat ground and mow often so the grass doesn't get too tall and/or thick, you could probably get by with a 6' mower. But, if the grass gets tall and thick and/or you have to go up any significant hills (15 degrees or more), you'll run out of HP. My 6' rear finish mower weighs about 650 lbs. So, that extra weight and the weight of the tractor, combined with mowing the grass will eat up HP going up a hill. You'll also need a fair amount of weight on the front to keep your front wheels on the ground, even on flat ground. My TC29D, which is close to 1000 lbs heavier than a TC21, doesn't need any weight on flat ground, but I need 65 lbs of weight on the front to be able to keep the front end on the ground if I go up hill. I'd go by the NH book and go with a 5' mower.