1 - Grab a similar item you like.
2 - Lay it flat and note: length, width and sleeve measurements in inches. 
3 - Look up your size on the product page chart.

 

 

 

 

NOTE:  It is standard in the industry that garment sizes have something called tolerance. This means that the sizes can always run +/- 1 inch or +/- 2.5 cm.