There is a paper size, half of a letter (letter is 8.5in x11.1 in), used a lot in enterprises and governor of Taiwan.
The actual spec of this paper size is 8.5in x 5.5in, which is called "中一刀" in Taiwan, means "middle one cut". I don't know if there is corresponding paper size in elsewhere.
I suggest to add this paper size in LibreOffice "Page" dialogue.
From the Wikipedia page on paper sizes:
... the 5.5 x 8.5 inch size would appear to also be known variously as: "Organizer L, Statement, Half Letter, Memo, and demitab / demi-tab". Version set to Inherited From OOo as this paper size has never been available in LO. Summary amended for clarity.
Setting version back from "Inherited From OOo" to what it was prior* (for bugs marked enhancement), for QA tracking purposes. My mistake. Apologies.
* In this case it was originally Master old -3.6 prior to 22.214.171.124 release. Set back to "Master old -3.6".