In the stationery world, there are two types of “A” sizing: the European paper-based “A” designation and the US system based on envelope sizes, specifically envelopes referred to as “A” size or Announcement envelopes. The A-sized envelopes are frequently used for greeting cards, invitations and personal stationery. The envelope sizes specifically correlate to the size of the flat or folded card inserted into said envelope. The most commonly used A-sized envelopes in use in the US are A-2, A-6 and A-7. There is also arcane reference to baronial or 4-bar envelopes which are occasionally called A-1.
The biggest distinction between Announcements envelopes and Baronial was that announcement envelopes originally had square flaps and baronial had pointed, triangle flaps. At some point though, envelope converters and paper companies made it possible to get envelopes with either square or pointed flaps.
There are several other envelope categories like business envelopes (the Classic no. 10 envelope holds a sheet of US letter paper folded in thirds), catalog envelopes, remittance, coin, etc. and, depending on the source you use, these envelopes can have square flaps or pointed flaps. Of course, you can have custom envelopes produced with different flap shapes and a different size but that just muddies the waters when talking about the standard options and the arcane naming systems.
The European “A” size is based on the paper size and the original uncut sheet of paper that was used. A-size paper starts with a full sheet at 841 x 1189mm (33.1 x 46.8 inches). When cut in half on the long edge those two sheets are considered A1, an A1 cut in half becomes two sheets of A2. Once an A2 sheet is cut in half to become A3 (297 x 420mm or 11.7 x 16.5 inches), the sheets become more manageable sizes and comparable to US ledger sized paper (11×17”). Then that sheet is cut to create the European A4 or standard letter-sized sheet (210x297mm or 8.3×11.7”). The sheet is continually cut in halves to the A10 measurement which creates tiny pieces of paper just 26x37mm or 1×1.5”. I can’t imagine that paper companies or mills find that size needed very often. I seldom see mention of stationery paper smaller than A6 or A7.
So, if you ever find yourself wondering who in the world would be selling A2-sized cards and envelopes and thinking “that sounds enormous!” Consider the likelihood that it is an American vendor referencing the A-size based on Announcement envelopes and not the A-size based in European paper sizes.