What this world needs is for t-shirt companies to put aside their differences, bury their respective hatchets, and come to the table to agree on universal definitions for what constitutes a small, medium, large, xl, etc.
Until that happens though, here's how OUR sizes work.
We print on NextLevel shirts, so if you own one of those, just buy the size you already know works. Easy peasy, mac-and-cheesy.
Otherwise, here's the NextLevel sizing chart for people who use numbers and stuff.