If you want to pop in and check how much disk space your Reseller account is using, you can do so right from WHM. You can also see how much disk space each of your individual cPanel accounts have from your cPanel dashboard.
View Disk Space per cPanel Account
To view the disk space allotment and usage for a specific account:
Click Account Information
Click List Accounts
The allotments for each cPanel account will be listed in the Quota and Disk Used columns.
View Disk Space for Entire Reseller Account
To see the total amount of disk space for your entire Reseller account:
Click Account Functions
Click Create a New Account
At the bottom of the page, in the Account Creation Resource Information section, the total disk space and bandwidth for the entire Reseller account is listed, along with the amounts allotted per package.