what is the smallest main gun on a tank?

A rifle caliber machine gun. The Renault FT-17 for example was armed with a 7.7 or 8mm Hotchkiss depending on the user. Through the 1930's the machine gun was considered the primary armament for most tanks.

I assume 20 mm cannon, but I recall seeing pictures of quad .50 BMGs mounted on a tank chassis as a mobile anti-aircraft weapon. They could have 20 mm's though. There were all sorts of things mounted on tank chassis. I saw a photo of some with four or more recoilless rifles used as a tank killer.

Others are correct. I saw films of early tanks with machine guns. They were used as infantry support and still are actually.

Ever ever? Early Panzers had a 20 mm gun, and I think that's the smallest that you'd find on anything that you'd recognize as a tank.

5.6 mm machine gun British Mark 4 1917