I will need more information as far as Scale, but I will assume that you just want to change the size of the circle. Click on the circle. You will see the circle's modify command ribbon. In this command ribbon is 'Diameter' and 'Radius'. Change the values to what you want. You can also specify the values at the time the circle is placed. Also, is you turn on 'Maintain Relationships', you can place a dimension on the circle and drive the diameter/radius by changing the dimension on the circle.