I went googling for sizes of Twitter Backgrounds so I could make my own. The information was so sparse and varied, I just experimented. I hope this guide helps you create your own custom Twitter background.
I went with the lower size because the average "webster" (my new word for a web user today) might have a smaller monitor and that's the size I used.
I work with two different size monitors and did a Print Screen to compare how my custom twitter background would look to others with perhaps similar screens.
pull over a guide to 5 inches from the edge.
This parameter is based on where twitter information ends and
my picture starts in my original screen grab.