The Seiko ColorPoint PS14 is a high-end color Postscript LaserWriter. It can use the Apple LaserWriter drivers, but also provides its own modified versions of those drivers. What the modified drivers allow you to do is access much larger image areas than the Apple standard drivers.
Since the printer uses rolls of paper, the available image areas can be quite large. However, simply selecting a larger print area in the Page Setup dialog is not enough. The image area must ALSO be selected on the control panel of the printer itself, and the printer must be taken offline to do this. It actually is not too difficult, but is a necessary step. If you don't do this, the printer will use the last setting, or the default of A4 size paper (roughly US Letter equivalent). Currently the Seiko ColorPoint has shipping versions of modified System 6.0.1 drivers. Their System 7 driver is not currently available at this time (June 20, 1991), but should be soon.
One reported problem with pre-release versions of their System 7 driver is that if any option is selected in the LaserWriter Options section of the Page Setup dialog, then the driver becomes confused about the appropriate image area for the document, and crops the picture. Removing the checked options in the Options solves the situation.