PoW is fuckhuge indeed. I'd attribute that to the dedicated core playerbase though, vilage, aaron, googer, hugs, and so on so forth. Were I able to keep up with it I'd probably still be in PoW (not that I haven't tried...).
Also this list is pretty flawed, I think RP's should be judged by IC alone as OOC can be watered down by chatzies and such fairly easily. Needless to say it's not a good indicator of length.
By IC the top ten would be...
1. Void of the Stars: A sci-Fi Nation Roleplay
2. Void of the Stars II
3. From Stardust, To Stardust
4. Precipice of War - We Write Under The Sign Of The Goat (RPG ONLY)
5. Civilization Roleplay: Rise of Races (The Tribes of Nova Mondial Want You!)
6. The Elder Scrolls: Rising Storm
7. The Darkness Descends (Fantasy Conquest RP)
8. Colonial Powers NRP (Reboot/New Thread)
9. Untold Empires: Legacy of the Old
10. The Iron Curtain: A Cold War NRP / Age of Rebirth (Post WW3) (TIED)
By length... Even with the colossal nature of Void PoW has some fuckhuge posts in its fuckhuge continuity, so I'd have to give it to PoW on gut feeling.