10th Anniversary T-shirt... only 200 being made

Is there a sizing chart as I manage to either go too big or too small on everything I buy online?

Bar One (who are producing these for us) claim these sizes:

Small - 34/36 Inch (chest)
Medium - 38/40 Inch (chest)
Large - 42/44 Inch (chest)
Xlarge - 46/48 Inch (chest)
Xxlarge - 48/50 Inch (chest)

American Apparel don't actually publish the sizes, but advise that the model in this photo is wearing a Medium.

http://s7d9.scene7.com/is/image/AmericanApparel/2001_black

I've found the Medium to be a comfortable fit and that they are fractionally larger than standard for the S/M/L sizes. I do not have an faithful comparison for XL/XXL.