I went out and found an 10' long board and placed it at 3' high to check the angle.
I would say 10' would be the very minimum ramp length for older Horses
Wood would be the preferred material and because of the snow, I would suggest some some strips (maybe 1"x2") across the ramp every foot for added traction.
Because of the height near the end maybe some sort of railing and also because of the height and the size of Horses maybe the ramp should be about 6' wide.
Just an opinion as I am not a Ramp Builder