Gedit

HOWTO:

Menu >> File >> Print >> Page Setup (tab) >> Paper-size >> scroll to - Manage Custom Sizes
Set both your required paper size and margins, rename the custom paper size and finally press the close button
Finally press the print button to set your choice

A couple of little things I did not know until recently that you could do within Gedit even though I have been using for some time:

Tabs can be re-ordered by dragging them around, as can be done in most tabbed browsers such as Mozilla Firefox.
Tabs can be grabbed and dragged out of the current windows to create a new instance of Gedit. However you does not appear that you can drag tabs from one instance of Gedit to another instance.