The setting you can use today is bytes/time which is kind of difficult when you are dealing with different resolutions and stuff.
A better measurement would be: bits/pixel/second, which automatically scale when size change.
Then you would need one of those settings per codec/compression method, so you could set one value for older codecs/compression techniques, and another for newer ones.
Also, it would be great to have a button/link on the detail page of an episode saying something like “try get a better version of this episode”