First off, do note that the naming convention of FF27, FF28, FFxx originally came from the old watchdeco site and was later adopted by Davidsen. As far as I know, Blancpain never used this system.
The DSN Blancpains are very good watches, but don't expect any of them to be a perfect replication of a particular model. I prefer to look at them as homages to some twenty years' worth of designs, incorporating features that are unmistakeably "Fifty Fathoms".
That said, if I had to choose the most accurate ones, they would be the Tornek TR-900 (FF21), the Milspec 1 (FF07), and "No-Radiations" (FF09). It's all about the thick bezel: The Tornek and Milspec were the only early FFs that had one. Thick bezels later made a comeback in the 70s, on the Bund and Polish Navy FFs. As an aside, the Polish Navy Fifty Fathoms is one of the most faked military watches, after the Omega Seamaster 300.
As far as I know, all of the Fifty Fathoms types were around 41mm, and either took 19 or 20mm straps. Movements on the early models were AS, but switched to ETA by the late 70s, I think. There used to be a detailed chart, but I'm afraid it's lost in an Internet sinkhole somewhere.
More pictures! This might help you in judging the size:
Here it is on mesh, on my tiny 6.5" wrist:
Sometimes it's on mesh, other times it's on vintage Tropic rubber:
I haven't tried vintagizing mine, but it would make a great base for a project. Kill a bit of the bright yellow on the markers, add some texture to the dial (but don't make it completely matte!), beat up the case a bit... It could be very nice, and I'm surprised so few have attempted it.