How to find diameter/radius of a circle? How to trace a circle?

I am creating a costume, and I need to make a circle through it to put it inside of a rectangle and to fit over my head (it's a Minecraft pig, if you're wondering). The circumference (the area around) I need is 32 inches. What is/how do you find the diameter and radius of a circle? And how to I trace a perfect circle that big?

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