Check out this man page:
paperconfig - configure the system default paper size
paperconfig [ -v,--version ] [ -h,--help ] [ -p, --paper papername | --force ]
paperconfig sets the system (or default) paper to be used by tools using the papersize >file. It can either ask interactively for the paper to use or be called non-interactively >by scripts.
When the paper size has been changed, paperconfig notifies other packages of the change >by running the scripts in the /etc/libpaper.d directory.
Print the version of paperconfig and exit.
Print help about usage and exit.
-p, --paper papername
Use papername if possible. If papername is not a valid paper, print an error message and exit.
Force interactive selection of the system paper even if the actual paper is a valid one.
PAPERCONF Full path to a file containing the paper size to use; this file will be rewritten with the system paper size.
Contains the name of the system-wide default paper size to be used if the PAPERSIZE variable is not set.
Directory of scripts to run after the paper size has been changed. This package puts no scripts here, but other packages may.
So you would run
paperconfig -p a4 in the commandline.