My mare is a 78 in Weathabeeta and an 80 in Greenhawk Shedrow brand.
I've found WIlton very helpful as well - nice people
Also factor in your shipping costs, especially if you end up needing to do a return.
Greenhawk blankets can be all over the map, so I'd not use them as a fit "standard"
(note that when you're googling to find sales, there are loads of tack shops that don't pay (Google) so it's worth your time to search the individual sites, also sometimes, a call will find a blanket in a backroom)
You might also look at the Amigo Bravo (new version with the liner attachments) system that utilizes the liners ... we did this with one horse last year, & were very pleased with the system - of course he's completely outgrown everything
My pony has a 66 Rambo Plus, a 66 Rhino Wug and a 66 Amigo. So I'd say the size would be the same. That said, my Rhino is my least favorite. It doesn't stay straight and it has a shorter drop than the other two.
So after speaking to the folks at Wilton they said the Rhino Heavyweight fits large like a Rambo. I currently have a Rambo protector flysheet in a 78 that fits well so I ordered the same size. Currently -24 here consistently over the past few days so hoping it fits once it gets here!