Hello Garrett_G
If you're not able to select the correct custom size paper from the driver, have you tried installing a different driver? If you are still having issues after trying these steps, I suggest that you call into our support department for assistance @ 1-800-821-2797.
I run a PrimeLink B9125 with an Oversized High Capacity Feeder and I cannot use 9" x 11" as a custom size through trays 6 & 7. 9" x 11" seems to only work through the tray 5 bypass tray but that doesn't hold enough paper for long run jobs. Is there someway to allow trays 6 & 7 to allow 9" x 11" sized stock? According to the user guide I should be able to run any custom size between 7.2" and 19.2" inches.