You don't want anything much over 4" for a big horse. Anything over that is too much work and can pull muscles and tendons. At 4" you can use it for basic RP work but it's too deep for riding. As far as size, 60' is norm. Big enough to canter comfortably and small enough to be able to put pressure on your horse in a nano second. 80' allows your horse to get too far away from you. As the horses get smaller they need less depth of footing. (a mini would die working in 4")
When I did mine, after grading, I put down 3" of road base, packed and rolled it. Watered it and packed it again. Added 4" of the largest grain sand (call it different thing different places). The road base makes a barrier and doesn't allow the dirt underneath to work up to the sand. It also sheds the water by stopping it from soaking down. The footing is always good, never have any wet spots and other than harrowing it, it has stood up to 10 years of use. To figure out how much sand you need, 1T covers 220 sq ft 1 inch. Roadbase is about the same.