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).
Many more in progress, but I don't want to spoil the surprises. Not that a grainy JPEG can really do that anyway. It just tends to spoil everything else instead. --- The noblest of all dogs is the hot dog: it feeds the hand that bites it. DoomBears!: http://www.youtube.com/watch?v=fU2xFxdK1t4
Also, I like the layout of your map, peach freak. Roughly how long did that take to build? --- The noblest of all dogs is the hot dog: it feeds the hand that bites it. DoomBears!: http://www.youtube.com/watch?v=fU2xFxdK1t4
Took me over three hours to beat, and that's not even counting the extra time added from reloading savegames! --- "Also, I just really can't think of any good ideas." -Koji Igarashi, Producer of the Castlevania series