My experience with building furniture items that require round tabletops yielded an idea that enables me to easily make tops of various sizes.
Take a wood yardstick and drill a nail hole at the 1" mark, making sure it is located in the center of the rule. Then, if you prefer a 6" radius, drill a larger hole (to accommodate a pencil) at the 7" mark, again centering the hole on the ruler. Next you can tap the nail into the center of your board blank (preferably into the underside of what will be the top) and push the pencil to form the 12" circle.
You can use the same ruler for any radius up to 34". If you have too many holes in the ruler or if they are too close together, simply use a new ruler. They are available at most home supply centers for just pennies. The only trick is to make sure the pencil fits tightly in the hole and is held straight when making the circle. After that, it is a simple matter of bandsawing the blank and sanding down to the pencil mark for a perfect edge.
