There are a few ways that you can.
1. Circumference- the distance around the circle.
2. Diameter- the distance from one midpoint to the opposite midpoint.
3. Radius- The distance from the center of the circle to any edge.
there may be other ways to measure a circle but these are the most used/most important.
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Since it is not possible to draw a circle using WikiAnswers, I will give you a link that has pictures of a circle with the parts labeled. http://www.mathgoodies.com/lessons/vol2/geometry.html