2015 is the centenary of the opening of the 15in gauge Ravenglass and Eskdale Railway, well more like the conversion from to 15in from 3ft gauge.
It seems a good opportunity to tie this in with the relaunched forum, as I would hope there is some interest and publicity in model railway press.
Here is my proposal then.
size to be 19 by 15 in(1915 connection) , no extra for fiddle stick etc(the extra 4in on base should be enough), and that is outside dimensions including backscene. This means it could be placed inside a piece of 15in gauge track as part of exhibition.
Gn15
Working , not static
At least one operational point or turntable (suggestion from Martin, I am not trying to promote my 3D printed wagon turntables, really...

Aim would be to show as many off as possible in one place, but could include video of those too far away. Also would aim to get something in model railway magazines, September issues as they tie in with the centenary event.
Back in 2006/7 Michael launched a challenge to build something in paper, and my effort by pure chance happens to be 19in by 15in. Luckily I still have photos, which I have put on one of my own webpages for speed and simplicity .
http://www.rue-d-etropal.com/Model-pics/Chateau-d-etropal/chateau-d-etropal-pix.htm
Hopefully this should show you can fit in quite a lot ,but it does not have to be continuous run, it can be like Carl's original module.