What size was the Cal of the machine guns in WW1?

There were several different types of machine guns in use. Most fell into the "medium" category, and had a bore typically ranging from 6.5 to 7.92mm. The heavy machine guns tended to have a bore in the vicinity of 12.7mm, but also includes the 14.5mm KPV machine gun. 20mm and larger are considered cannons.