I just did something like this a couple weeks ago with the background pattern, but not a texture. I had to make it 50x50 for my diagonal lines to work.
I would think that it can just tile whatever size image you give it, and that will look right as long as your image is designed to line up on the edges. Do you have a picture of what's happening?
(P.S. Good to see someone using Fireworks for layouts! That's what it's made for, and that's NOT what Photoshop is made for.)