I reckoned on the whole thig being about 6' / 2m long although the vertical tail bit of the sub would be modelled as a scenic piece at the stern end and the bows section would just be a solid rounded piece at the other end, so in total, the sub sections would be about 5' or so.
I was planning on 1/56th - its 574 feet long, so just a smidge over 10 feet, and c16" wide - of course, some of that would just be machinery, but i'm always interested in internal shots of subs etc - the challenge is to create something with the feel of a sub, but that allows for gameplay - ie narrow corridors vs being able to move/reach/or even see figures....