Traditional British paper sizes used a lot of royal terms, and I suspect that the name “Monarch” is an attempt to Anglicize a size which, in the colonies, is called “Executive.”

Personally, I like the traditional size name “Foolscap”…

foolscap (n.)
literally "fool's cap; cap worn by jesters," 1630s; c.1700 as a type of paper, so called because this type of paper originally was watermarked with a court jester's cap.

http://www.etymonline.com

n.