How to get the content size (in pixels, width and height) of the output in an output cell?
Make this button:
Button["Get size", Print@FrontEndExecute[ FrontEnd`ExportPacket[NotebookSelection, "BoundingBox"]]]
Then select the output cell and press the button.
It only works when the full output cell is selected (i.e. cell bracket), not just some content within the cell.
You might be able to move the selection programmatically to the output cell, then use this method (without the button).