Here in the US we just print posters at whatever size we want. We don’t have to rely on someone to have standardized the sizes of posters. Large–format printers often go up to eight feet wide, so you can print something as big as a wall if you want (and as long as you like, because they print on a _roll_ of paper instead of a sheet). Computers have made elegant ratios irrelevant.
But if you really think it’s important, then you can consider a series of sizes like tabloid, letter, and memo to be equivalent to A3, A4, and A5. Each is exactly half the area of the previous, and can be had by dividing the larger size in half along the longer side.
