My general sizing advice with NR is that sizing is very consistent generally across the styles (assuming no added optional liners) and quite TTS for use over a sweater : (1) take your normal US letter size, (2) size up one if you're planning on wearing on top of a jacket; (3) size up one again if you're planning on adding an optional liner most of the times.
So if I'm typically a US L (42), I will take a Large in NR. But I will size up to XL if I get a SB with liner. If I plan on wearing it mostly on top of a suit with the liner on, I'll take XXL. What matters is how you plan on wearing it most of the times. Squeezing it on top of a jacket from time to time isn't the end of the world if you typically wear the coat without a jacket. But doing it every other day during the winter will get annoying.
Start with your typical US lettered size, and order that in the Bumfreezer, Biker, Moscow, Single Breasted w/o liner, Double Breasted w/o liner
For the SB and DB, if you're ordering the arctic liner, order one size larger than your typical size. The liner does make them fit a full size slimmer.
If you plan on wearing this coat over a suit 80%+ of the time, consider sizing up again, with the exception being the Moscow. It has enough room in your typical size to accommodate one.
Possibly a 32, but I'd check the measurements of these against other 5-pocket style pants (e.g. jeans) you own and like the fit of. The tagged sizes on these are much more literal than most other jeans, with no vanity sizing.