'regular-sized' originates from the combination of 'regular' and 'size', where 'regular' meant 'usual or standard' and 'size' referred to the dimensions or magnitude of something.

'regular' and 'size' have been used in English since the Middle Ages, with 'regular' coming from Latin 'regularis' and 'size' from Old French 'sis' or 'size'.

Initially, 'regular' meant 'ruled or governed by rules', and 'size' referred to a set measurement. Over time, 'regular-sized' evolved to mean 'standard dimensions'.