If you are experiencing this error message "Invalid Maildirsize File" when attempting to try and change the email account quota in your cpanel. here is a solution.
1. Login to your website account's ftp with the cpanel username and password.
2. Browser over to the "mail" folder which you will see when you first login to ftp. The full path would be: /email@example.com (mydomain.com being your actual domain name)
3. From within this directory, you will want to delete the following file name: "maildirsize"
4. Once it's deleted, login to your cpanel account and try to change the quotas. It will then reform itself automatically and re-create the maildirsize file correctly.
You will now be able to change quotas for your email account(s).