It is quite unfortunate that we have developed international standard paper sizes but aren't able to agree on an international standard when it comes to poster sizes. Each country determines the standard size poster in its own territory. Some follow suit with the international paper size standards while others tend to follow the size standard that was once set by Britain. Here are the different sizes of the posters for some of the countries in the world.
Germany follows suit with international paper size standards. The names of the sizes of their posters are the same for their paper sizes. The German A1 size is approximately 23 x 33 inches. This is somewhat a standard in their country although you will also see smaller sizes such as the A2 and A3 among many others.
A slightly larger size than that of the A1 is the turplakat, which is only slightly longer. A much larger A0 size also exists, which is 33 x 46 inches and the A00, which has dimensions of 46 x 65 inches. The A00 is rather large poster that is made into two folding pieces.
Britain is one of the countries that have set standard sizes that other territories and countries have followed. Most of these countries were once under British rule, thus we can see how their standards have become adopted. Their one sheet is 27 x 41 inches and is in a vertical format. This is the size that the US modeled their sizes after. Nowadays it is replaced by the quad which is 40 x 30 inches and in a horizontal format. A double quad size also exists which is 40 x 60 inches and is in vertical format.
As stated earlier, the US patterned their poster sizes after the British sizes. However, as time moved on their standard sizes eventually changed. For instance, you can find US one sheets with dimensions as 27 x 40 inches and 27 x 39 inches. The US half sheet is 28 x 22 inches while the newer subway sheet is 59 x 45 inches, which is a larger poster typically found in their large metropolitan areas such as New York.
Italy and France
Italian poster sizes are somewhat unique. Their locandina size is 13 x 27 inches, which one would think to be a bit small. Their 2 foglio and 4 foglio dimensions are 39 x 55 inches and 55 x 78 inches respectively. These are the most common Italian poster sizes. You can compare these with the French standard sizes such as the petite 23.5 x 31.5 inches and grande 46 x 62 inches. Both are standard French posters.