The Circle of Confusion is the size of a blurry disk (blurred representation of a precise point which is not perfectly in focus) which a person would perceive as a blurry circle if it was any larger than that. In fact, CofC dimensions are 'manufacturer standard' size, and a person with 20/20 visual acuity would actually be able to perceive that size of blur circle as a blurry circle (the eye and brain of a person with 20/20 vision would NOT be fooled into thinking 'sharp point'!)

The size of the CofC has nothing to do with FL...if your eye detects a blurry circle, it would not matter if that blur were captured with a 13mm lens or 130mm lens or 1300mm lens.