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Biggest and longest are two different things. What are you asking?
Most detailed? Most number of sectors? Longest time to complete? Most enemies contributing to a long play time? Largest in grid-size?
For longest levels in terms of time to complete and amount of different rooms, the best I can say is probably stuff Eternal Doom. Or perhaps MAP20: Misri Halek from Alien Vendetta. Some of the maps in Alien Vendetta are also quite large and detailed. Most of the maps have a ton of enemies too and that's partly what makes them so long. MAP26: Dark Dome on UV for example. It's not that "large" but the sheer number of enemies makes it take a while to complete on the first couple of run throughs.
Knee Deep in ZDoom is probably among the most detailed of the WADs out there but the levels don't necesserily take an extreme amount of time to complete other than probably Z1M7 or Z1M10 (the secret secret map).