What is the normal dilation (or diameter?) of the common hepatic bile duct and the common bile duct?

Question

I had a recent MR MRCP W WO CONTRAST with these results:

BILE DUCTS: Mild intra- and extrahepatic bile duct dilatation. Common hepatic duct diameter 9 mm and common bile duct diameter 8 mm. Common bile duct tapers smoothly at the ampulla, and there is no stone, stricture, or mass. 

I don't know the average diameter of these ducts for comparison. Can you help?