Not so fast. Your 72mm answer seems closer to the truth. The frame is 65mm wide, but the image circle has to cover the diagonal, and not just the width. In fact, I suspect it's larger than 72mm--maybe closer to 80. Excuse me while I trudge down to the basement for my calculator.
"If you always do what you've always done, you'll always get what you always got."
--Variously attributed to Moms Maybley, Mark Twain, etc.