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.
The Oblivion Gate thing would be excellent, epecially if it was the same on the 'you go in, things can come out' spectrum. It would mean you could build a general violent mob farm around a portal, if their standstill, and when the health and mobs on SMP are fixed, you could create a survival arena!
Developed the puzzle branch for the map Diversity 2, and recently released a puzzle map called The Present Factory. I later put the guts in a map called "Valley of the Kings", which I'm too lazy to link.
The creeper would SSS, but then stop as I held him in my arms. I would look down upon him, and his frown would turn into a beautiful smile.
He would look up at me.. A single tear would come from his eye, and he would say "Thank you".
I like the idea of placeable portals making the door themselves, duplicating downward like water without the lateral flow. That way we can make them as simple or ornate as we want. I'd be going with a smooth stone "stargate" look myself.