Its from the middle of the chest to the tail (along the sides), but your best bet is to try other peoples rugs on and see which size fits best. And try it on with a sheet underneath when you get it, so you can take it back if it doesn't fit.
Thanks guys, think I may splash out on a cooler rug for when he's hot / wet. Musn't get addicted to rug buying, musn't get addicted to rug buying........!