You will not be able to fool the printer using custom size media from the trays. You will need to selects custom size settings from front panel/drivers and feed media from the multi-purpose/bypass tray. To read specifics to set up this feature you have a couple options, use the user manual on the CD-ROM, you can find this by browsing it. If the printer is on the network and you know the IP address you can access the user manual on the printers hard drive by putting the IP address in the address window of your internet browser then click on the support tab when the printers web site opens. You can also probably download the user guide off of Xerox's website.