I had a lot of trouble with paper sizes in LibreOffice always going back to US Letter, but I found paperconf command-line utility to be of great help:.
Check out the man page forpaperconf
Read the default Paper size
paperconf
This returns the contents of the papersize file
Set the default paper size
sudo paperconfig -p a4
This creates or overwrites the /etc/papersize file
Example Usage
You normally want the paper set at A4 size, but take the case where you are working a project with index cards, and you want A6 to be the default for the time being:
sudo paperconfig -p a6
When done, and you want to go back to A4 your usual A4 default
sudo paperconfig -p a4
