All the gear here at Online Cycling Gear is aimed to match typical western race-fit sizing.
Cycling apparel does typically fit a size smaller than casual wear, so for a snug race fit, it would be best to order a size up from your typical T-Shirt size.
To help with this, every listing has a sizing chart to guide you on the best size (look for the SIZING CHART link on each product listing under the size selection).
Generally, the larger you are, the bigger step up in size you will want to make and if you find yourself in between sizes, it is advised to always choose the larger of the two.
If manually measuring and your stomach is bigger than your chest, use your stomach measurement instead.