Ok, so the horse will have 2 fenced acres to use part of the time? [If so, I'd make sure he/she gets to spend plenty of time in the bigger area.] I wouldn't go smaller than 25x50' for a drylot, if they will be spending quite a bit of time in the area (that's 1250 square feet, and probably a bit small for a 12H Shetland (it's the approximate size of my 36" mini stallion's run, summer he has a small grazing area in addition to his regular run). [800 square feet might be enough, but bigger is almost always better. 800 is 20x40', so my recommendation is a bit bigger, but that little bit could make a big difference in his happiness.]