Personally I would say no. Sure they can fit in but they always end up spreading the muck around all over the place unless they are a rare model and poop in one corner!
I have a 16h in a 10x12 box and although she doesn't seem cramped and has never got cast, she could probably do with a bigger stable. She is VERY messy and I think her smallish stable contributes to this.
I've always gone by 10' x 10' to be a pony box, 10' x 12' to be a small horse box and 12' x 12' fopr large horse and 12' x 16' to be foaling box.
Personally i would say a little too small for the size horse you are thinking of putting in there. Would increase risk of getting cast and less mobility in the stable say if they were in over the winter but at the end of the day you would only really know by putting the horse in the stable and judging for yourself!
I had my old 17hh TB in a 10 x 10 ft box which obviously wasn't ideal but it was all we could have at the time. He was absolutely fine in it, even rolled in it every day and never got cast.