I would leave as many trees as possible, with some open areas as well. Keep in mind that you can always remove trees if you deem it necessary, but you can't put a mature tree back in a short period of time
Are you looking to make all 7.5 acres into pastureland? Or are you planning on some sort of development- barn, house, arena, etc. If you're going to develop the land into anything other than pasture, then I'd start getting an idea of what you want to do and plan on leaving the rest as woods/pasture.
After you've roped off any "development" areas, I'd plan on clearing a fairly large area towards the front of the pasture for grazing, with several patches of shade trees around the open areas. If it were me, I'd try to keep a fair amount of the wooded area as well. Consider fencing off the tamed pasture area if you don't want the hassle of keeping toxic plants out of the woods where your horses are. Put a gate between the two areas, so you can keep them contained or let them out. It'll also allow for easy access to the back if you want to maintain a few winding trails through the woods.