I agree with SoundGuy. I've got a 6 foot on my TC33D, because a 5 ft would not cover my tracks with R4 tires. I also like to do a bit better than just cover my tracks... makes it easier when you are trying to get right up to the edge of something. In some situations, 6 foot is a bit big for me, but I just take smaller bits when I hit those situations. For maintaining a gravel drive, 6 foot fits my 33 HP very well.
I'd imagine 6 foot would do well for you in almost every application. If your primary use was maintaining already constructed roads, you might get away with bigger.
John Mc