Given room and food N. gigantea do get large, but honestly? No larger than some of the standard type day bloomers. Mostly they are leggy so can take loads of room.
Granted the pictures here are not N. gigantea, but the closely allied N. immutabilis and N. violacea. I let immutabilis get large, but I have purposely kept N. violacea small. While violacea doesn't look as impressive, it is doing what I want, which is giving me pollen and seed.<g>
And yes Scott, you are wrong by a 'Juno' mile.<g>