When UCP requested that our senior design team investigate cheaper AT buttons, we started with a commercial button as a reference:

(representative image from xkeys.com)

They were pretty large, so we considered shrinking the design in order to save on 3D printer filament, and made some test prints to compare:

Eventually we settled on a diameter of about 10cm, which provided a middle ground between surface area to hit and printable size:

Were we to do it again, I'd want to focus more on obtaining many commercially available buttons and having ours match the apparent standards. This picking-values-out-of-the-air approach leads me to believe that our original design is less interchangeable than it could be.