I have found they run a tad small. I can wear a medium in a lot of shirts if they aren't short but large is border line of being to small in the Sitka t shirts. Couple bowls of ice cream over a weekend and I have to wait a few days to wear one.
I’m XL in about every tee made and an XL in the Sitka tees fits me perfect. Every generation of their tees has varied slightly, but I’ve taken a XL in everyone of them.