In 8th edition my biggest game was a Triumph & Treachery game with 5 players, 3000 points each so a total of 15000 points. Very fun, I love the manipulative side of these games.
Biggest game I´ve ever participated in was during 6th edition, we had played a map based campaign from General´s compendium, 14 players in total, we had a big game to end it all with various bonuses for the leaders in the campaign, in total it was somewhere between 30000 - 35000 points played out on 4 ping pong tables (side by side), in the end the evil side won due to the bretonnian deathstar lance couldn´t break a unit of 18 chaos warriors (defensively equipped, bsb with stubborn banner standing next to them), we swamped the deathstar which was worth over 1500 points in total, this made the good side have to adapt their tactics which they failed to do.
As TheREALricksalamone says above, this was also my largest game ever. The Ultimate Alliance of Absolute Power. Good side dwarves, dwarves, empire, bretts and tomb kings vs. orcs and goblins, woc, woc and ogres. Good side 5 players at 5,000 points each and evil 3 players at 6,000 points each and the last at 7,000 points. This made 25,000 points per a side. 9 players total.
The game was heavily documented on this forum. Here is the main and lengthy thread. Lots of pics. I include just a few below.