Normally, the website would have a sizing chart with exact measurements for each size; unfortunately the company Team Meat are using seem to be relatively new and haven't put one up yet.
So, you could contact them at firstname.lastname@example.org
and ask for the measurements for a couple of sizes.
Or, you could use another site's sizing chart as a rough guide, like http://www.thinkgeek.com/help/sizing-info.shtml
or something. I personally have no idea how accurate this method would be, but from a quick google around, the sizes do vary from site to site.
edit: just realised you can get some shirts from the Newgrounds store, so here is their sizing chart:
found at http://www.newgrounds.com/lit/faq/