If you want to increase/decrease the FTP Quota or to use an "Unlimited" FTP Quota but also want to set a limit, then follow these steps to modify the FTP User quota.
1. Log into your cPanel account.
2. In the "Files" section, click on "FTP Accounts" Icon.
3. Under the "FTP Accounts" list, click on "Change Quota". 
4. Click on the "Change Quota" button.