Briefly, the "small c" Retinas have small eyepieces/reticles. The "Big C" models have much larger eyepieces and brighter viewfinders. Plus, they have three framelines visible, which some photographers find to clutter the viewfinder.

The non-folding Retina IIIS and IIS have large, bright viewfinders very similar/nearly identical to the Big C models. Depending on the lens mounted, the Retina IIIS should display the correct framelines. The Retina IIS is a fixed-lens rangefinder with a 45/2.8 Schneider-Kreuznach Xenar and has just one set of framelines.

I think eyeglass wearers all suffer from the same problem when using a camera. The eye must be set further back from the camera's eyepiece, almost ensuring that you'll have a tough time seeing the entire viewfinder. That's true of SLRs, rangefinders and simple viewfinder cameras (but not most TLRs).