No absolute answers, as it depends on teh movie and how well it compresses.
However for a 90 min movie you'd be looking at 1-1.5GB for SD, and 3-4GB for 720p HD, more for 1080p HD.
The size of the rentals is the same as the rental/purchase in iTunes so you'll get a feel for it there.
Also if worried about bandwidth you might be better off renting in iTunes then streaming over the LAN under the AppleTV computers menu. AppleTV has not storage - if you finish a movie and want to watch again or use AppleTV for something else having only watched half a movie with a view to finishing later, AppleTV redownloads the movie using more bandwidth again. So if you rent a movie and watch it 3 times you might use 12GB for a single movie of 4GB, but only 4GB if watched via iTunes.
The traffic across your LAN will not affect your monthly plan, AppleTV only uses a tiny amount of connection bandwidth to authorise playback of a rental etc.
So best options - rentSD in iTunes or look for a better broadband deal.
AC