For default text printing, the file /etc/cups/lpoptions contains all the info for each printer. To change the default font size (for plain text files), change the cpi (characters per inch) and lpi (lines per inch) options.
By default, they look to be set as cpi=12 and lpi=7. To make the default font bigger, make these values smaller. I usually use cpi=10 and lpi=6.