Test ride.
Not just around a car park. Proper ride if you can. It’s the only way to be sure unless you can judge from specs and reviews, ideally with experience (and better if you can return if you don’t like it). I did the latter with my recent 456 build but then you can’t test ride them and they’re cheap. A rig like a Nomad is expensive and you really should ride it first to see.
For what it’s worth though 5’7″, 31″ inside leg and test rode a medium, and ended up buying it. Doesn’t mean much stating what fits me as it’s also up to how you like your ride.
That said I sometimes find the frame a tiny bit short for me, but I’ve read this is natural design with the Nomad. On the other hand the 456 is quite a long reach and going by geometry specs based on what I know of similar bikes, I ended up with the small which is just right. Still a little longer than the Nomad. Reach and seat position is bang on, whereas the Nomad is slightly off the back.
All said, slightly too small is better than slightly too large, but small Nomad is very small in my opinion. Just my opinion though. Again, test ride!