The easiest way is to measure the height of the stamp in millimeters (perf tip to perf tip). A stamp 36mm high uses size 36 mount. The actual mount itself is 5mm taller than that. Hope this helps.
Thanks for the info - the scale will be very useful. I had read that an extra mm or two should be added to the stamp dimensions to ensure proper fit - which is why I posted my question.
Comments
Check out this link for a stamp mount scale. You cam print it out:
https://www.kenmorestamp.com/media/wysiwyg/Collecting/Guide/StampMountScale/Stamp-Mount-Scale.pdf