I have thought about this logic, and could see it being useful. If I am in town, select a mount that fits through doors, if I am in the wild, select from the whole list. I know that I would still need to dismount to enter if coming from out of town, but I seem to get the largest dragons each time I am trying to do business in town.

As far as I know there's no way to find out how big a mount is automatically, or anything about its 3D model.

What you can do is make a group for SMALL mounts and use that group in a rule using the Resting condition.