Looking at the picture of the bike, all signs point to it being a bottom-end bike from a Japanese factory: the laid-back frame angles (indicating that the bike was built using the jigs for the factory's one-speed and three-speed frames, which would have been the bulk of their business), the square-cut lugs, the need for a claw for the rear derailleur, etc.

The seat post size therefore is almost certainly 25.4.