This is a pretty unique piece...but man, I just don't like the "Big Date" implementation by Mondaine. It doesn't look good on what could be such a clean, crisp face (and is, sans date, on many of their other watches.)
You have A627.30303.11GOT and A669.30305.11GOT, respectively 40mm and 30mm, according to their site:
https://www.mondaine.com/watches/official-swiss-railways-watch/evo/a669-30305-11got.html
If you look at the photos you see 30303 engraved on the back of this watch, so 40mm seems correct.
So if you want the smaller one just buy the ladies version which they now also offer here.