Mahindra 7520, Mahindra 3215HST, Case 580 extendahoe, Case 310 dozer, Parsons trencher, Cat D6,
Don't know how heavy your tractor is, info says 4000 -7300lbs. If you are on the lighter side, I would suggest that you only build a 6 footer. Either way, try to get it up in the 1000lb range and be sure that the sides are no less than 60" in length and 72" would get you a better finish grade. Is your main use going to be for roads or landscaping? Some guys prefer for the blades to be straight for landscaping purposes, I don't think that it really makes much difference myself.
I know that there seems to be roughly 3 different heights used for these graders. 12", 18" & 24" sides.
Good luck with your design & build, don't forget to post pictures, we all like pictures.