I've had sex with Donkeys, (My Favorite among Equines) Shetland Ponies (These little guys are mean and love to bite you) Miniature Horses (Also like to bite but not as mean tempered as a Shetland pony) and 6 different breeds of full sized horses. IMO Full sized horses are truly too much for vaginal entry, I would think the same for anal. When they push in and get rock hard and flare inside you there is such an expansion of their size that is can rip you a little. This happened to me and it truly caused me to stop having sex with them for 3 months before I got the courage to try it again. Some breeds are larger than others and the one that ripped me was a larger breed(Tennessee Walker) but my interests here have turned to Donkeys. These smaller guys are still equipped well - they have 10 or more inches past the ring but the thickness and flare is smaller and much easier to handle.
When they are inside the feeling for me is a very full feeling with a pressure pushing outward. They gush their sperm out in a few bursts and it is maybe 4 ounces of it. Then they withdraw. Actually I do prefer sex with dogs as the feeling when they have their knot inside is incredible, the throbbing and warm flood of their sperm excites me much more than with a horse.
I only have sex in the missionary position with horses, and the horse is tied right up against the wall to prevent them from pushing too much inside by moving forward. I lay on a bale of hay with blankets on top and the horse and I are stomach to stomach. Both my sister and I only do it this way and we think it is probably the only safe way if you're going to try sex with horses. Both of us don't do anything anal so I can't tell you what that might be like.