Being trained as a scientist, I finally broke down and did some measurements to answer the question at hand with numbers. So, Here is what I came up with. I used a cloth tape, leaving some room for error, but the general pattern should still be good and measured all the fins I have. All but the newer Sporasubs are 44-46. Keep in mind that some footpockets, like the Imersion and old Dessault are quite soft and elastic, while others are hard and rigid, like my new Omers, the 46-48 Sporasubs, and the Picassos.
length width height cross section
old Dessasult 27.7 10.8 1.9 20.6
Dessault HD 28.4 10.2 3.2 32.3
Imersion 29.0 12.1 2.5 30.6
Picasso 27.7 11.4 2.2 24.4
Omer 28.4 10.2 2.9 29.0
Sporasub 29 11.4 3.2 36.3
The Sporasubs are 46-48, so are expected to be larger in cross section. They don't have a country of manufacture, and are slightly different than the Dessault HDs in shape, and they're really stiff. Plus, they don't say Dessault anywhere. Their downfall is the plastic piece the screws attach through. It sticks up into the footpocket so far that it irritates my toes when wearing them.
And the reason I have so many pairs? We're a long way from the nearest freediving shop, and I still haven't found the perfect pair to stick my SpecialFins into. None seem to be really designed for wear with 5 mm socks and wide feet. ; (
Howard