You change the size drawing style of all observations in the current data set by clicking the Size button in the primary console or the temporary console and a size palette like one of the following two will show up:
If the Shift key is held down when the OK button is clicked, the selected size will also be used as the default drawing size for any data sets loaded into Argos in the future.
Size palette is modal.
How to toggle between the 2 modes of drawing circles is explained here.