I have a smaller apha mare. She wears a 34 'girth and has a med size build
Measure from the middle of her chest around the side of her body to the middle of her tail. Whatever your measurement is, add a few inches to that and order that size. Remember to add a few inches or you will more than likely have a blanket that is too small.
It really depends on how high she is and what her measurements are.
Here's a website that explains how to chose and buy a rug for your horse. To get to the measureing part scroll down.
http ://www.countrywidefarmers.co.uk/pws/...
http://www.equisearch.com/horses_riding_ ... Read and then follow the directions. It has nothing to do with girth size.
You would have to measure from her chest to her bum and then add an extra few inches:)
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Hope i helped
