There is also B4, which is 353 x 250 mm. Not perfect, but you can more easily see how they got 250, as B0 is 1414 x 1000 mm.

A series paper billboards would be interesting to see.

I wonder what it would take to make paper with more precise measurements, say to the nearest 100 micrometres.

Pretty sure the geometry of the image is off, despite the numbers being correct. The even sizes are slightly too square and the odd sizes are slightly too narrow. It makes me feel like somebody accidentally compressed the entire image horizontally before overlaying text over it.

Below is my attempt at remaking the image:

You’ll like B series paper then