There are so many questions one should ask when considering horse ownership. Your question, the size of the horse, while it is a consideration, is one that is less important than other questions.
Do you have horse experience? And I mean more than just hanging around barns and other people's horses. Do you have any experience in their care and upkeep? Grooming, cleaning and maintainance of their feet, general daily care- cleaning stalls, water bucket upkeep, manure management, dietary considerations, etc.
Do you have proper facilities to care for a horse? Do you plan to have them on your own property, or board? If you plan have them on your property, do you have property management skills needed for horse ownership? Having them on your own property is nice, but a lot of work. Also, if you plan to have a horse on your own property, you should avoid having just one. Horses are herd animals and need the company of other herd animals (horses preferred, but can be donkeys, mules, goats, etc) to feel safe and happy. If you were to board them, how often will you see them and spend time with them?
What do you want to do with your horse? Do you want to ride? Will your horse be a pasture pet? Regardless if you want to ride or not, horses need A LOT of time and attention to bond to their owners. Do you have that time you can give your horse so that they'll bond to you in the way you desire? If you want an amazingly close bond (which I strongly recommend before you become sexually active with your mare), you need to spend DAILY time with them, and you will need to learn proper horsemanship techniques to make sure your mare is well-behaved and easy to handle, not just for you, but for the safety of others.
I don't ask all these questions to discourage you, I ask them to know if you are truly prepared for the task that is equine management. I have had horses over 20 years, and they take daily effort and lots of time. The more you put into your horses, the more you'll get out of them. They are truly a joy to be with when you have happy, healthy, contented horses that WANT to be with you... and that doesn't even include any of the sexuality. That's just the cherry on top to an amazing relationship with one of the most amazing animals on earth!
So, while I understand the desire to enjoy sex with a mare (and I truly do, as I consider my primary riding horse my equine wife and we are intimate often), please think of what the horse that you are looking at NEEDS to be happy and healthy. Remember that their needs should always come first, as they depend on us for those things.
If you would like to talk about horses more in-depth, please feel free to DM me.