Sold by piece by weight. Sizes may look similar depending on shape/thickness. Each stone has been weighted. 
XS: 10g to <15g
SM: 15g to <20
MD: 20g to <25g
LG: 20g to <30g
XL: 30g to 35g
Stones cranked up by Jay Essex and Diana R. Anaya (Dianimal)