Step 1: Open Web site admin panel (https://:10000)

Step 2: Login to admin user account with password

Step 3: expand 'Servers' , click on 'Virtualmin Virtual Servers'

Step 4: click on your domain name (e.g. example.com)

Step 5: Expand section 'Quotas and limits' . You can check your web site quota (include database / mail / web site / FTP)

Step 6: you can click on button 'Edit Mail and FTP users' to check personal quota

Step 6: click on E-mail ,which you would like to check personal quota.

Step 7: Expand section 'Quota and home directory settings' for personal disk quota.

You can setup as 'unlimited'. Quota cannot excess  the main disk quota.