Add a custom pagesize to printer options

[Hugo Segovia]

First, hello to all
I use an app which uses a pagesize known in Windows world as "fanfold standard german", or something like that, which I never found on any of the printers I installed. No problem, I can use "lpr -o PageSize=Custom..." to achieve the desired effect.
But I find doing it over and over annoying. So I want to know: How can I add a "custom paper size" (let's say, "FanfoldCustom" :-), which I can later set to be the "default print size"? Should I edit my printer's PPD?
Even better, can you point me to the right documentation for doing that?