Customizing Roll Paper Size on P5000

[etevansfineartstudio]

My P5000 is working perfectly. There's a roll setting in Photoshop for 16" Roll paper which cuts off at 20". So, I've created Photoshop templates that allows me to print many shots on Epson roll paper, 16" width. However, if I want to print only one shot, the paper cuts off at 20". That means if the shot is a 16"x9" ratio, I cuts off the sheet at 20" --leaving 11" of blank paper. Does anyone know how to create a custom roll size that is 16"x 11", for example? Or 16" x any other number?

Please don't respond unless you have tried to do this and succeeded. When you hit "print". there's a "create custom size" field which I have tried, but it is ignored. I'm on a Mac.