I've run a 39 floor dungeon with each floor containing between 5 and 80 rooms, and with multiple paths to every location... it was incredible... till the party was killed utterly on floor 13...
I used a random map generator, and randomly placed starcases up and down the thing in random locations to random floors pretty much. Pretty simple process, but making sure everything lines up appropriatly took months. It was a fun as hell hack-n-slash though... all the carnage.
Let me recommend you stick to EL's exactly 1 below the group level in big dungeons, because at level encounters and 1 over resulted in a TPK when the group was ambushed at night when they tried to spend time in one of the primary coridors...
Make it a maze that even you don't know the way out of, you know it's there... but you don't know the path yourself. That way you have more fun, in my humble opinion.
Also, I recommend having themes that add a lot to the story that pervade every area... scribbles on walls that have nothign to do with the main plot, but if decoded reveal some truely ominous information and such...
Actually, if only I could get the entire underground dwarven ruins of Morrowind on map. It's an entire underground dwarven continent, that your group won't be able to get out of till thy can travel etherreal through five hundred miles of solid earth... it would be fantastic. There is little information on Morrowind around, so I can't tell you to go looking for it.
Your going ot have to do at least a lot of your own work, so buckle down and get some grandious overview done and fill in the spots in between campaign sessions...
I'm ranting I think?