Category strips on vb3x boards are usually 40px in height, they commonly use the url tcat and use repeat-x top left in the CSS attribute to create the length.
This height increases when you add a forum description that places underneath the forum category titile as seen on here at vb.org.
As you are using an image I think this is going to be a test each time to see how it sits on your board.
I would suggest doing a size of 40px X 900px and put your image in the first third to see how it sits as you also have a 'wide' skin to, depending where you want it!
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