Width of a box blade should 99% of the time be wider than your tractor's track width (outside to outside of tread) - my older 7' Gannon box (right at 1100 pounds) gets used on my Allis 160
TractorData.com Allis Chalmers 16 tractor information
which is heavier than your tractor, but NOT 4WD - when it starts to seriously dig, I have to keep my heel on the differential lock but that extra weight REALLY helps it cut. The 7' Gannon is about 5" wider than the tires on both sides, wouldn't want it any less.
Woods (NOT the model mentioned in previous post) now makes the Gannon versions (if you can afford one)
EA has some really well-built stuff, but unless you don't need it til NEXT year, you might be able to find a used older Gannon quicker and cheaper. It all depends on what you need to do with it whether you get something that LOOKS like the "real thing" or actually IS the real thing - My old 1100 lb. Gannon on the back of a Case 580 has done
900 cubic yard cut/fill projects (made a 1 acre sloped area into a level spot) in a 3 day weekend.
Prior to having my OWN gear, a
45 horse RENTED Kubota and a 500# box blade took 2 DAYS just to smooth out a DIFFERENT smaller sized project AFTER I rented a JD 450 Dozer with 6-way blade to move about 100 yards of material from behind my barn to make ANOTHER level area.
My point - it helps to KNOW what you want/need to do, THEN you'll know what to look for... Steve