What is the most common logo size used for Visa, Mastercard, AmEx, Discover, JCB, and UnionPay on standard bank cards?

Each card scheme has its own branding guidelines and how to put up the log. More of less they are the same to the casual eye, but each scheme has put forth very rigorous rules on how their logos, etc. should be placed etc.

For example the most common size is that of the VISA logo, 9.75mm x 20.5mm


Others have their own standards. You may want to consult with their brand identity guidelines to gauge the exact size.