Ive got the Amigo Mio on both my heavy Fell and my sec b show pony, they seem to fit anything!
I do have to extend my fells neck cover a little, but a piece of string does the trick just fine.
The Rambo sweetitch rugs, shires fly sheets and the weatherbeeta ones are all too tight on the neck and chest, so I dont even consider them!
The Bridleway ones look like they might be quite roomy though, but I havent tried one...
I think it depends a little on the size of the rug too, I frank is cross over from pony to horse size necks at a 5'9. The amigo neck wouldn't go anywhere near him his rhino turnout the same I bought a 6' neck to go with it (fits fine) but the neck on his rambo flybuster is ok, wouldn't mind it a little larger. I don't think his neck is that big though!
Mark Todd Ultra Combo, its got elastic an elastic strap at the top of the neck so there's plenty of give there, although he's Welsh and slim so I don't really know if it would fit something chunky.