My Weatherbeeta Saxon sheet fits my bulldog mare surprisingly well. Otherwise, I'd say go for the "westerny" brands, like Big D or even Weaver Leather makes a few blankets. I just bought their midweight blanket a few weeks ago, it fits my mare like a GLOVE, with plenty of shoulder and butt room.
Tell a Gelding. Ask a Stallion. Discuss it with a Mare... Pray if it's a Pony!
The best fit for my QH boy was the weatherbeeta Taka. I have one in every weight for him, including a sheet. It's fits him well - here's a pic of my QH boy, Jax, in his blanket. He's the one on the Left - in the blue and red plaid.
My other big boy on the right, an Appendix, is wearing it in the winterberry plaid (green and burgundy).
As you can see - they are nice and roomy through the shoulders.
Rambo QH and Amigo are both made by Horsewear Ireland. The Rambo QH line seems to fit pretty true to size, my size 74 mare wears a size 74. The Amigo line fits differently, you are supposed to subtract 4" from your horses normal size. When I did, and ordered a 69", it was too small for my paint, so I sized up to a 72", which is still smaller than her normal size but not the suggested 4" that they recommend.
When I measured my BS Paint for a Rhino I did the subtaraction and got him a 78 even though he always wears an 81. It was too small (super tight on his big shoulders) and I had to send it back to get the 81 that he always wears. He also has a Wheatherbeeta Orican that fits well in an 81.
Another Weatherbeeta user here. It is about the ONLY sheet out there that fits my bulldoggy QH with a chest wider than the barn wall. I buy the Freestyle blankets and sheets for her, which are even roomier yet.
I think my prob with my Orican Freestyle is that I was going by Rambo measurement rules and bought a 76 instead of a 78. She wears a 75 in a Rambo. They don't make this particular Orican any more. I really like it, but could use a bigger one.
Well, now I'm seeing this in an ad:
Please note Weatherbeeta blankets run larger than some other blanket types. They are measured from the center of the chest around the side of the horse to the end of the hindquarters rather than to the tail.
If an 84 saxon fits perfectly but is too short (miniskirt, damages his sensitive male ego) how is an 84 waterproof Amigo sheet going to fit? He's very huge and bulky and such (half Percheron, half quarter horse- so kind of like a QH build on steroids).
I also got an amigo sheet (summer sheet) for the little guy. I think I ordered a 75. A 76 usually fits him OK but is a bit too large, he's a narrow little twig of a dude.
It is my experience that regular Rambo products (not the QH line, haven't used those) do not fit horses with large chests/shoulders. I had a Wug for an old QH I had, and although it fit him great everywhere else, he couldn't move in it because it was so tight on his shoulders. Had the same experience with my stocky Appy mare. My draft cross wears Rambos but his bulk is in his butt, not his shoulders.
My Appy mare has done extremely well in a Dover Northwind, and I also recently bought her a new WB Orican that seems to fit her well.