If using x3, edit the following file:
/usr/local/cpanel/base/frontend/x3/mail/popsinclude.html
and modify
input type="text" name="quota" value="250"
Change the value from 250 to your new default. This will modify the default quota server-wide.
find /home/USERNAME/mail/DOMAIN/ -type f -name maildirsize -exec rm -rf {} \;
This would remove the quota files for each mailbox, which is different. The article shows how to set the default mailbox size within cPanel when creating new accounts. By default, this is 250M, but can be set to whatever you like, using my instructions.
how do i restart cpanel in order to make sure the changes were made?