Comment by jccalhoun
Yes, in the USA letter size is the standard. A3,4 don't exist. It isn't confusing because I would guess that more than half of all people in the USA don't even know that letter size isn't the standard everywhere. I was probably in my late 20s before I found out that Europe doesn't use the same size paper as we in the USA do. I can remember exactly once that I encountered it in the wild (I was at a conference and someone from Europe had some handouts).
The European sizes exist in the USA if you want them, you just have to order them from a print shop or supplier.
Or you can get whatever you want - I wanted B4 paper to print a booklet (or B3 maybe) and I just bought a ream that was larger and had a print shop slice it down to B4. My US laser printer was fine printing onto B4.