The bigest game I ever played would have to be the final battle of a campaign I designed, the Battle of Fort Augusta. It was a narrative-style campaign and the winner had already been decided, but, as usual, I put a last game in there for fun. Every single Black Templar from two armies up against every single model in three Ork and one Chaos players' armies. We were in a wonderfully built Chapter Keep, with our forces, and two thousand points of armed fortifications, our six thousand points of Templars against nine thousand points of Orks led by two thousand five hundred points of Chaos. This, of course, was all worked out after the battle. The objective was for the Templars to have at least one man alive after twelve turns, which was, quite naturally, near impossible and had to be, given the Orks and Chaos had already won. We lasted eleven turns, and the last models left were my Emperor's Champion and High Marshal (Marshal with +1 wound) in combat with two Warbosses, six Nobs, fifty three Orks and a Chaos Dreadnought. Truly an epic scene, and the very incarnation of all that is Black Templars.
For those who want to know, the two had charged into combat five turns ago, out the front gate of the Keep, with a dozen Sword Brethren, into a Waboss and a huge Nob bodyguard, and between the two of them in that one combat racked up six hundred points of kills. For their sheer awesomeness, I built a copy of each, painteds them as bronze statues, and had them mounted on the gates of the Keep. Sure, it belonged to the shop, but the owner had come up with the idea in the first place.