i was told it's two foot off your rug size so if your horse wears a 6'3'' your exercise sheet should be a 4'3'' not sure how accurate but is prob a good place to start
You measure them from the whithers to the top of the tail and then round to the nearest 3" as the sheets come in 4'3", 4'6", 4'9" etc...
You can't really just know from the size of the horse alone. My mare is a 14.3hh but takes a size 6'6" rug, my friends 15.1hh takes a 5'9" rug. Best to measure to be on the safe side.
my cob wears a 6`3`` rug.i had a 4`6`` excercise sheet that went under the saddle and it really annoyed me,it was like a mini-skirt on her and used to pull flat on her withers..i have now bought a 4`9`` wrap around one and it is just right,she is a chunky type.
OP - it depends on personal preference also, i'd say around 4'6" would fit your mare but maybe best to measure to be on safe side! I prefer my sheets a bit on the snug side as i find the ones that technically fit properly tend to flap about as soon as a wind picks up!