6 mm outside diameter by 4 mm inside diameter. Sold in one meter (1 meter equals a little over 3 feet) sections.
Incorrectly pictured in the Rancilio Epoca 1 Group diagram for providing water to the hydraulics, instead use R_1332.
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.