In order of sheer size I think the 1/48 kits go like this (starting with largest):
B-1B (Revell)
C-130 (Italeri, but Testors used to make one)
B-29 (Revell/Monogram)
B-17G (Revell/Monogram)
B-24 (Revell/Monogram)
B-25 (Revell/Mongram)
I haven't found anything bigger than the B-1B, and that beast is enormous (I've looked). My brother built one once...good grief! I built the Testors version of the 1/48 A/C-130 Hercules about 12 years ago, and that thing was crazy-huge. It also weighed about 8lbs (about 3.6kg).