Wacom Cintiq 21ux display resolution/size

I have an older (2009) Wacom Cintiq 21ux. I've got it working nicely with my iMac , which was a lot of work, all except one thing. the graphics on my cintiq are not set at the right resolution and they run off the page, are large and pixelated. When I go to system preferences>display> the "color LCD" options (which is what they're calling my cintiq) don't have options for the scaled resolution? Any idea why?