Step 1 - Log into cPanel
Step 2 - Click on the "Email Accounts" icon.
Step 3: Click the "Change Quota" link beside the mailbox that you want to change.
Step 4: The Mailbox Quota form will appear under the mailbox. Enter the desired size or select the "Unlimited" option * (Recommended), then click the "Change Quota" button. The mailbox quota will then be updated with your new settings.
* If the "Unlimited" option is selected, the mailbox will be allowed to use as much disk space as it requires until the limit of the entire hosting package is reached.