If I am reading this right, your horse will be kept in a 10x40 area, correct? I would at least triple that to be 30x120, and even that doesn't have a whole lot of area to move around. 10x40 feet is enough room to jog for a few steps, but nothing more for a horse. Horses are made to move around and run, not be kept in a little pen. Also, you would probably have to move your horse 1-2 times every day. I would fence as many 80x120 areas in as I could, which is a quarter acre. 200x200 feet is roughly an acre.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown