I would -like- it to be friggin' huge... but it's a fast travel dimension, gonna get a lot of use, so it's probably gonna be small.
If you could make your own portals that would be cool, but Notch hasn't said which one he's gonna do yet. (make your own or randomly placed)
It would also be cool if you could make the shape of the portal, like Hotec said, a new portal "block" that when you touch it you get transported to hell. Then you could make small and big ones, of whatever shape and size you want. I would definitely build one like in WoW, the Dark Portal, all huge and dark and menacing. Too bad you can't have statues in Minecraft, that would be awesome.