I guess you're going for realism, so I'd say the place to start is... same focal length as used to make the photos - presumably about 90mm for a "normal" focal length, roughly 11 diopters. If you want to test out the concept, just use the original lens to inspect a contact print.

There was some discussion of "proper viewing distance" of prints several months ago, in this thread:
http://www.apug.org/forums/forum52/1...d-shots-9.html
In my post #83 I attached a sketch from an old "how to draw" book, demonstrating an idea of how "proper viewing distance" works with different image sizes.

In other words, my suggestion is to NOT approach this as an attempt to fill the field of view, but rather to go for the "proper viewing distance," then use optics allowing the eye to focus at this distance.

I don't know how to get both eyes to work together for this, so perhaps some trial and error. If you don't already have a set or two of closeup lenses, perhaps you have some low-end enlarger lenses to play with.