How to Build a large stereoscope?

I am an artist and i am embarking on a project where I want to scan old stereoscopic negatives and enlarge them, then build a large mirror stereoscope to view them with. I am hoping to use the mirror stereoscope blueprint I found here

http://www.funsci.com/fun3_en/stscp/stscp.htm#6

but the design does not specify how big the images can be or if theres a formula for how big or far apart the mirrors should be.

Ideally i would like to make the images appear more life-size or to try and give that impression.

Can anyone please let me know if they have done this before or know any pointers?

Thanks a million
Ross