From Lunarpages Web Hosting Wiki
If you need to modify the quota (storage space) for a customer, and do not want to change the entire package's quota, this can be done individually per each account. To do so from the main menu of WHM click on Account Functions, then on Quota Modifications.
Next click on the domain name you wish to modify the quota on. Wait a moment, and information on the account will appear to the right. Now click on the Modify button at the bottom of the page.
The next page will show their current quota, and the box next to it is where you can type the new quota in megabytes.
Enter the new quota in the box, then click the Change button.