I think the largest folder I've ever owned is the Ontario Retribution I, designed by Jerry Hossom. I traded it recently because it was simply too big for anything I'd need in the field or camping.
I've put a lot of thought into this recently because I've decided to trade and/or sell most of my large folders, mostly those over 5-1/2" closed. I don't carry or use them much at all.
So, just for the heck of it, this and the next post will have pics I've made recently of a bunch of my large folders; to me, "large" is anything 5" long closed or longer. However, I've had several big chunky folders that were not as long, but larger in other dimensions.
Since I had pics already made, I figured I'd post them up here for a size comparison. (I'm constantly surprised at how much smaller or larger a knife is 'in real life' once I've received it.) Shown with a 6" machinist's rule and a 2AA-cell MagLite flashlight for scale.
U.S. Schrade LB7, 5-1/8" closed. Not the largest on the market, but the first large folder I had (in high school), and the one I compare all my others too.
Case #6265 and U.S. Schrade #25OT two-blade Folding Hunters, 5-3/8" closed.
Remington (Camillus made) R1253, 5-3/8" closed. The longest traditional type I have.
Spyderco Military, 5-1/2" closed.
~Chris