confused about clam sizes

[prestage3ek]

When I go online and look at clams, what do they base the size on. LIke I will look at a clam and the description says its 3 inches, well 3 inches isnt very big, and the clam in the picture looks more like its 8 inches. Can someone explain?

 

[skinz78]

Clam size is based on the measurement from end to end on the shell.