The best way to size a shirt is to compare it to one that already fits.
Lay it down flat and measure to compare; this is how we measure our shirts.

For our cotton shirts, such as our silkscreened 100 Yen or Tokyo Mashers:
For our poly shirts, such as our all-over pattern Cabs and Bullet Hell:
For our ringer shirts, such as our Sweats / One Credit: