Hi Francky, also in addition to just typing in the radius value while you are in the circle command, you can alter an existing circle's radius value by clicking on the Radius field in the properties panel that shows up when you select an object:





Similar to the circle drawing command, you can click on the "Radius" label on the edit size popup menu to toggle it to diameter mode. Also when the menu is launched you can just start typing in your new radius value without needing to click in the field first.

- Michael