Add support for roll paper

Hey,

how do I add support into Print settings for roll paper (endless paper)? I have installed all drivers and all little bits, and printer works. However, I have a roll paper, and I want my printer to print long images on continuous roll. What happens now is that, since the paper size is preset to A1, printer knife just cuts the image at A1 size, and does not allow me to print really long images.

Everywhere I looked on the internet people just said "Set paper size to Roll Paper" but I do not have that option. It is just not there. All options for paper size are regular fixed sizes (A4, A3, A1, ...), there is no Roll Paper.

How do I add support for that?

Printer is HP designjet 800, MacOS X 10.8.3, with all the latest updates.

Thanks.