I had a similar problem where sending envelope jobs via MS word envelope creater on the PS driver was recieved as 8.5x11 on the printer(not your model). When I switched to PCL driver it worked
I am attempting to print 7.2 inch by 5.2 inch envelopes from Stamps.Com and from the Stamps.Com software - same thing happens both times. The printer errors out and says it is waiting on the 8.5x11 inch paper resource (and sometimes, with no apparant changes, it'll ask for 8.5/14 inch paper).
The 7.2x5.2 size is set up correctly in Stamps.Com's software, in the printer's own interface, and in the Xerox driver; I even set up the default paper size in both the bypass tray and in tray 1 to be 7.2x5.2 envelope.
However, each time, the print job errors and it says it is waiting on the 8.5x11 resource (and again, without any changes, sometimes it asks for 8.5x14).