Sorry to get back to you so late... here's a posting of 3D printed stereoscopic postcard:http://www.thingiverse.com/thing:1792658
The dimensions I used are 3.5" by 5" (which I think is your average small postcard).
And, the important thing is that the centers of the two images are 63.5mm from one another. How large you can get away with depends on the focal length of the lenses you chose. The shorter the focal length, the smaller the image needs to be to fit within what you can see at that focal distance. Hope that helps!