I believe you open Fax Console, click Tools, Fax Printer Configuration,
Printing Preferences, and change the Paper size. It looks like it's
referring to a printer used to print incoming faxes, but it's the print
preferences for the fax "device", which is also considered a printer.
You can also open Control Panel, Printers applet, right-click Fax, click
Properties, and you'll see the same console open.