I would say either a 6' or 6.5' would probably be good. Are you getting it to just do your own driveway, or are you going to be plowing for others as well? I would say the 24-Series would probably be youur best bet, but if you're only doing your own driveway, the 18-Series would probably suffice.
You need to go with the 6'8" Series 24. The next size down is only 69" and will never clear your track width when angled. The 7'6" is only about 40 lbs heavier and would give better clearance at the sides of the rig. And I think that both of them are lighter than a 6'6" or two meter from the other makers.