In an adult, what size (in mm or cm) differentiates a small, medium, and large unrepaired atrial septal defect?

ASD: These are really arbitrary, subjective terms, but i think of small as less than 8mm, medium as 8-16mm and large as greater than 16mm. However, the bottom line as to whether or not an ASD needs to get closed by catheter-based device or surgery hinges on whether the right side of the heart (right atrium and right ventricle) are enlarged.

Ok, varieties: Small can be from pinpoint patent foramen ovale to a centimeter or so. Large can be absence of the inter atrial wall! most secundum defects are 2-4 cm.