...
When you enter in the desired values and click OK, the aperture boundaries are added to the GUI as dotted lines.
You may return to the aperture dialog box to readjust your values if you wish. When you are happy, click "done" or type "q" to exit the aperture definition GUI and return to the pipeline. Another xatv GUI may pop up immediately after to confirm your object placement; you can ignore this instance and quit out to proceed with the object extraction.