This warning is about your webspace, not your email space. Every Frontier customer starts off with 25MB for their webspace and 25MB for their email — depending on the chosen service plan!
When taking into account the size of your webspace, there are two folders involved. The first is the public_html folder and the second is the public_ftp folder. If the total size of the two folders is over 25.0mb you will receive the auto-generated message from us.
Two weeks from the time the email was sent all files will be removed from the public_ftp and public_html folders if you don't clear enough files to get under the quota limit. To clear files from your webspace, you will need to transfer your files via FTP (File Transfer Protocol) to your space on the Frontier servers using an FTP program such as CuteFTP or WS_FTP and delete files as necessary.