Teefury

Hey I bought my first shirt from teefury not that long ago, and I was debating over getting a mens small or a medium. I always wear men's smalls because they generally fit me. However my mum got me a medium because she said it was better too big than too small. A time before I ordered it I got the Arkham city Game, that came with a Shirt, but the shirt only comes in Large, to make it less exclusive. We measured up the shirts and it turns out the Medium was the same size as this Large shirt. I don't know if it's because Teefury is american or something and sizing works different than to the UK, but any suggestions on what size I should buy next time? Tomorrows shirt is something I really want to buy but unsure of the size.

Thanks.