My opinion only.
I think 14 is perfect for working on the go. 13 too small even on the go. 15 is great but it’s unnecessarily large. 14 is the sweet spot.
Here’s the great flash of insight I recently had: If I’m not on the go, I shouldn’t be working on the laptop monitor at all! I should (and will) get a decent 24" monitor, keyboard, and mouse and work on those.
Just saying. Thanks to Purism for picking a good thing, focusing on this one thing, and (hopefully - we’ll find out) doing it really well.