View Quota and Disk Space Used
From Lunarpages Web Hosting Wiki
On your Webmaster Reseller account, to view the Quota (storage) assigned to an account, or to see the disk space used by the account, log into WHM. Then click Account Information > List Accounts.
You will now see a list of the accounts you have set up, along with a search bar if you have many. Scroll to the right to view the column that shows the Quota (in megs) and the Disk space used by the account.
If one of your clients has gone over their quota and you want to manually change the quota for just them (and not change the package quota) go back to the Main WHM menu, click on Account Functions, then on Quota Modification.