This is a really interesting discussion. There is a lot of personal preference when it comes to viewfinders - and size isn't everything. My Hasselblad housing has a massive viewfinder that you can look through with both eyes simultaneously! But I still prefer a standard housing.
I think the thing to avoid is a housing viewfinder that does now shot you the whole frame. A housing that makes you move your head from side to side to see the whole frame is a disaster - but there are still plenty of housings out there like that. Make sure you take a mask to the shop when trying housings.
Another important factor when comparing cameras is viewfinder brightness. Some have better focus screens than others and this can make a big difference in the dark depths of the ocean. My Nikon D2X is signifcantly brighter than my Nikon F100 and I actually prefer the view through the D2X viewfinder (in the housing) to the F100 (despite the F100 being bigger and full frame).
I am still umming and ahhing over getting the GS viewfinder for my Subal. I have used it on the D70 and it makes a massive difference to the view although I am not sure it makes any difference to my photography.
There is such a lot of personal preference in viewfinder choices. People have different eyesight and have different preferences when in comes to composing images and relying on AF.
Alex