First, use the Circle Tool to draw a circle in an empty plot window. Don't worry about its size or even if its a perfect circle, but you can edit its other properties (color, line width, etc.) if you wish. Then right click on the circle, select Object Control and give it a name if it doesn't already have one. (I'll just call it "circle".) Save it as an object file by opening the script window and entering the command:

draw -n circle -f save circle;

Open another plot window and retrieve the circle with the command:

draw -n circle -f read circle;

Use its object properties to position and size the circle. The following script line places the center of the circle at X=5 and Y=10 in axes units:

circle.x=5; circle.y=10;

The next line sets the diameter of the circle to 500 in layer units:

circle.width=500; circle.height=500;

I'm not sure off the top of my head how to convert from layer units to axes units, but there must be a way. You'll find several other object properties that can be changed with script commands.

The only reason for saving the circle to an object file is that there is no way (that I know of) to create a circle from scratch with script commands. If you draw a circle and save it to file you can use it as often as you like.