I think that "Like the way it is done when I import data from an excel-sheet " refers to the sized circle/shaded circle option on the Data Mapping Wizard. Not sure, but that is the only "fixed/free diameter and color" circle I can think of that is not a draw object.

If you plan on using this without the Data Mapping Wizard see the help on DisplayDataMap method and the DataMapType constants "geoDataMapTypeShadedCircle" and "geoDataMapTypeSizedCircle."

If you just want the circles without importing data, I think you will need to use the shape object.