International = in
American = " (double quote).
Since ChipGuide is in American English; a safe method of "inch" would he inch/inches, I would think.
The entrance pass measures 2" x 4"
The napkin measures 6-inches square.
I would not accept I, I, in or anything other than the word inch or " and I would think the International measurement for chips, jetons and plaques should be millimeter (39 mm) or (60 mm x 40 mm) or in cm.