After testing numerous tuba makes and models, two shank sizes were developed to meet the needs of a wide variety of mouthpiece receivers: American (US) shanks fit most American made and styled instruments, while European (EU) shanks fit many European made and styled instruments, with the exception being most 6/4 CC tubas, which require the EU shank. 

American (US) Tuba Shank Measurements:
Top: 15.44mm/0.608"
Bottom: 13.31mm/0.524"
Taper Angle: 1.41 degrees