Verify if you can find the following code in your dovecot configuration:
Code:
mail_plugins = $mail_plugins quota
protocol imap {
mail_plugins = $mail_plugins imap_quota
}
plugin {
quota = maildir
}
If it's not the case add it manually or try via custombuild:
Code:
cd /usr/local/directadmin/custombuild
./build set dovecot_conf yes
./build dovecot_conf
Once the quota module is enabled in your dovecot it should be visible in the roundcube webmail..