Horse sheet size?

I am looking at getting Joe a sheet that I can put on him when it is in between cold and warm. He has like no hair but will sweat with his stable blankie on some days. I saw this one on Stateline that was cheap and nice looking only thing is, it is an 80 and Joe is generally a 78. I tried someone elses turnout blankie that is a 78, and it fit him but his butt was a little exposed sooo im thinking an 80 would fit just be a little baggy??? He is kept in a stall so its not like he's going to run around in it lol Opinions please?? Thanks!!
Oh and Joe is about 16hh TB but he has a bigger booty lol so I was thinking that would take up extra space lol? Opps forgot the link!
http://www.statelinetack.com/itemdy0...1=SLT723300+80

I think the size 80 would work well on Joe.

Ok Thanks! I think I might get it before it gets sold out!

Yeah, if the 78 left his butt exposed, then the 80 would probably be perfect. Plus, it's cotton...if it's too big, just wash it in warm and fry it in the dryer! My horse fits great in his 81/82 blankets, but I also have a few 84s that I got real cheap that he wears just fine (turnout & stall). The bigger ones just get in the way of his pooping sometimes and so I have to wash the crusties off the back edges a lot. (eww--sorry for the gory details)

PS - you probably want to pick up some rear leg straps for the blanket (web says not included). Without them, when he lays down in his stall, the back end can slide to one side and he could possibly step on it when he stands up. Then who knows what he could get into with a half blanket hanging from his side. (Or, he'd probably just somehow get poop on it!) =)