Interesting. When I select User Defined, a popup window appears that contains a list of previously defined paper sizes on the left and on the right a space for a new name and then below, places to define Paper Width and Paper Height and Units of Measure.
I'm not sure what is going on with the 3800 driver. I'll let someone who owns that printer lend you a hand.
Yes , have tried that but must be doing something wrong as even though I choose the User Defined box I cannot find anywhere to put in the dimensions.