Sold by piece by weight. Sizes may look similar depending on shape/thickness.
First pictures were taken with the stones soaked in water to showcase it's true color.
Each Turritella stone has been weighted. 
XS: 5g to <10g
SM: 10g to <15g
MD: 15g to <20g
LG: 20g to <25g
XL:  25g to 30g
Stones cranked up by Jay Essex and Diana R. Anaya (Dianimal)
Pictures taken both in natural and artificial lighting