HP Deskjet 5850 Color Inkjet Printer - Manual (Page 148)

setting a custom paper size
Use the Custom Paper Size dialog box to print on a special paper size.
defining a custom paper size
Follow these steps to define a custom paper size:
1. Open the Printer Properties dialog box.
2. Click the Paper/Quality tab.
3. Click the Custom button.
The Custom Paper Size dialog box is displayed.