It's specific (US Letter https://en.wikipedia.org/wiki/Letter_(paper_size) ) - half of which is reasonaby close to A5, but I'm not sure why the "odd" qualifier.

I guess odd because it’s really only used in North America. I’ve never seen a piece of letter sized paper although I’ve heard of it (first after deciphering the weird PC LOAD LETTER error on HP laser printers which is extra confusing if you don’t realise that letter is a paper size).

Suddenly, after 25 years, that message makes sense! Thank you!

PC is Paper Cartridge, iirc.

It is almost the same as A4 paper.

And yet different enough to sometimes cause issues when they get mixed and matched (e.g. A4 document on Letter printer, or vice versa).

It's almost like a misaligned a4

Ah!

Unless you know about the particulars of US paper sizes, it sounds like they mean 'half the length of a letter that you might write to someone' which does sound odd. What letter? What about? My letters may be one paragraph or three pages.

One could also interpret it as referring to a letter of the alphabet, just to add to the confusion.

"US letter size" - https://en.wikipedia.org/wiki/Letter_(paper_size)

Letters are usually written on A4, if you fold that in half you get A5.