Just to confuse things, im 5.10, i own a small of which im very happy with the size.
I rode a medium, 60mm stem, felt roomy and great, but as i wore more and more tired, my arse was moving further and further forward on the saddle.
My preferences are to stand 95% of the time (the SB66 is the best out of the saddle pedalling bike ive ridden and that’s comparing to 100mm bikes), i only really sit and pootle on road sections or really long, dull uphills. So even if it isn’t roomy seated, it doesn’t really effect me.
Just to put how big the medium SB66 is, it has a 24″ TT which is most companies L/XL, so even if you’re old school and used to 120mm+ stems, you should be able to achieve all the breathing spce you want on a medium.