What does 57mm diameter look like?

Answer 1

A 57mm diameter would be the distance across a circle, measured from one point on the circumference, passing through the center, to the opposite point on the circumference. In practical terms, it would be slightly less than the width of a standard credit card, which is about 85.60mm x 53.98mm. Visualizing it, a 57mm diameter circle would be slightly larger than a US half dollar coin, which has a diameter of 30.61mm.

Answer 2

Diameter means that you're talking about something that's round, either like a ball or like a pipe. 57 mm is just a little more than two inches, like 2 1/4".