Manage Disk Space in WHM
From Lunarpages Web Hosting Wiki
This is how you would manage disk space for your accounts using Web Host Manager (WHM). Doing so will give you the ability to lower or raise the storage space for any account managed under your WHM account.
The first thing you would need to do is login to WHM. From the front page, click on "Account Functions" and then "Quota Modification". This will take you to the Quota Editor.
This page will list each of your domains hosted on your dedicated plan, the space used currently, and the overall quota. You can change the text box for the quota to lower or raise the disk space to what you need. Once you are done, hit the "Change" button.