Hi Neil, thanks for the measurements.
Well I don't know if 4"X6" is super big but it is big enough for a few days fishing. My saltwater box that I carry is 4"X9"X2 1/8". The Days Worth box seems like a good idea if you want a large fly box.
I have not used the Orvis Dropper Rig Fly Box but I have thought it would be good for saltwater fishing. I guess it would be good for float trips also. I may get one for my pontoon boat.
Orvis Dropper Rig Fly Box