If you want the block to be inserted into other drawings, start a new drawing. Launch the bedit command. Choose <current drawing>. Draw your circle(bubble) with the center at 0,0 and make its diameter 1/2". In the properties palette, in bedit, with nothing selected, choose "annotative" "yes".
Create attributes if required with the height set to the plot size. Close bedit and save and close the drawing. To control the size of the bubble when inserted, use the annotation scaling system.
Architect, Registered NC, VA, SC, & GA.