I'm another that says you can't tell by looking!! And you can't go by the mare's height!!! I was present when my vet examined an under 27" mare and a 34"+ mare, and she said the under 27" mare was very roomy whereas the 34" mare wasn't. I also have a 27.5" mare that is VERY roomy inside according to my vet, and I hauled a 32" chunk of a mare in for a friend, and my vet said the mare was very tight.
All of my maiden and new mares are examined by my vet to see how much room they have. I won't breed one that she says shouldn't be bred, or that is really tight.