Ok, I have heard of this method for drawing a really large circle, I am attaching the link. I would practice on a large piece of paper first. you can take two pencils attach a string to each pencil. Someone will have to hold it down in the middle of the circle and then go out as far as you want the size of the circle to be and pivot around the person holding the other end of the string. Here is the link, may make more sense.
http://www.ehow.com/how_2239326_draw-circle-string.html