When drawing a circle, the command is automatically asking for the Radius dimension.  To create a circle with a Diameter, you have to press D on the keyboard.  There should be a way to change the default dimension that the command is actually asking for.  I don't have to draw many circles with a radius dimension.  Mostly arcs are the only things dimensioned with radii.  If there is not a way to do this, then take this as a suggestion to add it to the software as an enhancement.