Uniqlo "Premium Cotton" crewneck (these are great, nice fit and keep their shape pretty well; nice and short too) in S:
~18.5" pit to pit (after wash)
mjk henley M/38 - in the bamboo material, which is awesome. Love this, will get some more when I'm back from my holiday
My other mjk stuff also fits really well - slim and just the right length.
~19" pit to pit (after wash)
Haven't been to a Gap for a while, but saw these at the weekend and bought a couple - "Gap Fitted" (in S). They also had a stretch "skinny" one that seemed to fit almost exactly the same... The Fitted ones are nice really thick cotton though, feel like they should last a while. I could easily have bought the XS in these, but they were a tiny bit short - there's no way I'm an XS...
~17.5" pit to pit (new)