A friend of mine sent me (from the pub - lucky chap) the above photo of a watch being worn by one of his "associates." As you can see it's a Railmaster Chrono and judging from the shot, it looks to be rather large. I know Omega made XL Railmasters but I've never seen an XL Chrono before. I assume they're not that wide-spread?
It is 42.2 mm, same size as full size Railmaster. There are 3 size of Railmaster (which based on Aqua Terra): mid size 39.2 mm, full size 42.2mm and XXL size 49.2 mm. They were a common production (not limited edition) but less popular than the standard model, which more resemble the original '60 Railmaster or the XXL model which generate buzz because of its size back then, but looks ridiculous now. More info : http://www.omegawatches.com/press/press-kit-text/463
Most kind of you to reply, thanks for the clarification on the different sizes and the useful link. As you say, 49.2 mm really is pushing it from most point of views.