How about this: there's no legal requirement you label your products in any given unit, but if you choose 'inches', they have to be real 'inches'. You can use your own made-up unit, say 'oldnavies', instead -- just don't call them 'inches'.
(Even minarchists usually find laws -- common-law or legislative -- against commercial fraud acceptable.)
I thought there were bodies of government[1] out there that specialize is certifying companies who use 'inches' and want to mean inches. If you are not certified, I don't think there is a guarantee that what they say is 100% accurate.
(Even minarchists usually find laws -- common-law or legislative -- against commercial fraud acceptable.)