Sold by piece by weight. Sizes may look similar depending on shape/thickness. Each Amazonite stone has been weighted. 
 
XS: 10g to <15g
SM: 15g to <20g
MD: 20g to <25g
LG: 25g to <30g
XL:  30g to 35g
XXL: 35+g
 
Amazonite: Increases vibrations while cleaning problematic frequencies. Often used near computers and other RFI electronic equipment.  (Book II of Creation series)
Tourmaline and Kyanite work well with it
 
Stones cranked up by Jay Essex and Diana R. Anaya (Dianimal)
Pictures taken both in natural and artificial lighting