Yep good idea!! Thank you!Welcome.
You'll need to measure. My lad has changed sizes recently after getting fitter (not shorter).
I did it by getting some baler twine and cutting or marking the length of twine from the middle of the pony's chest to where the rug would end, then measuring the twine. As with anything, measure twice...ponies move.
ETA: I had no option but to use twine, I didn't have a tape measure which went over 7'.
(He's come down from 7'6" to 7'3")