Email to fax - send legal size documents

I have fax to email operating on my server and it works just fine. However, it cannot send anything other than a letter size document. If a legal size is sent, it only sends the bottom 11 inches, cutting off the top 3 inches.

The syntax is to use the following as the subject line in an email:

[keyword] fax_phone_number

I'd like to explore the possibility of adding a switch that tells the fax server to send the pages as legal size, so something like this:

[keyword] -legal fax_phone_number

Is this easy/hard/impossible?