1. Sign in to Virtualmin (see Account logins and important URLs).
2. Once you have logged in you should see an overview of your Disk and Bandwidth usage on the main Virtualmin page.
3. For a more detailed breakdown of your disk usage click Administration Options in the left-hand navigation then click Disk Usage.

From here you can view disk usage by Home directory, User, sub-server (domains under your main/master domain), and database by clicking on the appropriate tab.