My Advice: Keep It Simple Storyteller.
In my campaign, most of the dungeons are less than a dozen rooms. This includes actual dungeons, caves, buildings, what have you. A massive 30+ room complex, like the one in the back of the PHB, should by rare and dangerous, as the resources needed to build it are immense and exploring it can be a chore that can take up to 3 or more sessions. I usually leave things of that size for a climactic end to a current campaign cycle, where big things are afoot and the PC's have found the source of the problem.
I tend to like to make sure that the characters spend as much time in an adventure outside of a dungeon as they do inside in order to keep things interesting. A little roleplay, a little combat each session, to provide everyone with something to do. As an example, in one adventure, the PCs explored:
1. The barrow of a desecrated Paladin (three rooms, one trap, two wights and a spectre).
2. A Church run by a corrupt priest (9 rooms, the priest, his two adepts, two adult teifling children of the priest and his demon lover, and an orphaned hunchback rogue kept in the attic)
3. An abandoned bathhouse (4 rooms, one grey ooze, one 'possesive' ghost)
4. A wilderness encounter (one owlbear, 6 wild elves protecting it).
5. A jailbreak (one room, one half-orc ravager).
6. Finally: an underwater cave complex of a dark sea goddess (9 rooms, 3 Merrow gaurds, a scrag, a sea hag, 9 Dark Oceanids, a giant squid and a water genasii priestess).
And believe it or not all of these exploration/combat encounters were linked to the greater story and all of them were seperated by enough investigating, NPC interaction and general roleplaying that the whole thing took six 8-hour sessions to complete. Basically, one exploration event per session.
While the plot is complex, the individual exploration areas are KISS simple and served as a reward for great roleplaying. None of them lasted long enough to bore the players with 'bash-kill-loot' repitition, all of them were different in a unique way, most included at least one non-combat encounter/solution, and all of them advanced the plot in a way that the players actually gained more out of completeing them than mere xp and goodies. That is the way I like to design them...
