+1 on the Busse NMFBM. It is the largest chopper I own, and the amount of destruction it can dish out is downright scary. The NMFBM is like the freak child of a sledgehammer and a straight razor- combining the hitting power and indestructibility of the sledge with the sharpness of a razor. You can sever tree limbs with it all day and then shave with it.
But the NMFBM only has a 11.5" blade. In terms of size, there are plenty of machetes and kukris out there that are bigger. But once you get above 12" or so, I don't think you're really talking about "knives" anymore, you're talking about swords.