I haven't measured the film opening, but my Foldex 20 is a nominal 6x9---I haven't noticed that it's significantly different in size from my other 6x9s. Of course, it's not a "good" camera by anyone's definition, except in durability---it's not just built like a tank, it's built like something you could use to *break* a tank.
I believe the Steinheil lens is a Cassar, which is a perfectly good triplet, certainly better than the Octvar lens on the Foldex 20 (which I'm pretty sure is just a meniscus). As far as I can tell, a Foldex 30 is basically a Foldex 20 with a Vario shutter and a (focussable) Cassar, which sounds like it should be a perfectly serviceable basic 6x9 folder.
The dubious viewfinder is common to folders generally, I think. The Foldex 20 has a nice, clear, comfortable viewfinder whose main drawback is that it has almost nothing to do with what will be in the picture! Frame generously and crop, I think, or just be willing to say "oh well" a lot.
-NT