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. 

European (EU) Tuba Shank Measurements: 
Top: 15.85mm/0.624"
Bottom: 13.77mm/0.542"
Taper Angle: 1.37 degrees