5.25" disks are 5.25"/133mm along each side, making 5.25" a perfectly good description, in my view, for an object at this sort of scale. The disc inside must be

5.25" disks are 5.25"/133mm along each side, making 5.25" a perfectly good description, in my view, for an object at this sort of scale. The disc inside must be 130mm in diameter, the case is presumably 3mm larger, and the whole thing is named after the size of the case. 5.25" = 133.35mm. I expect you could actually make a perfectly usable 5.25" disk that was exactly 5.25" wide... Apparently 8" disks are 8"/203mm wide, no doubt for exactly the same reason, and with a rather similar result.