I am no expert, but from what I can see it looks like it fits well around the chest and sits nicely infront of the wither and doesnt look like it will slip back behind wither. It does look a little long, as in goes a bit further over his rump than usual, but I don't necessarily find that a bad thing if the rest of the rug fits well. As for length down his legs, again it looks a little long, but not ridiculously so. If this is a rug you've bought, I'd be quite happy to use it, but if it is one you are just trying for size, you probably could go a fraction smaller.
Maybe others might think differently though :)
I think it looks too tight on his shoulder he's going to be constricted moving around. It may not be the size that is incorrect it just might be the style of blanket. I've noticed that the cheaper blankets just don't seem to fit as well as the more expensive ones