For those with fixed width forums, how wide is the standard size? Is there some sort of rule of thumb? I want to make a header image but I'm not sure how wide it should be.
The rule of thumb is to set aside 15 - 20px from each side off of a 800px screen size. So the total width of the body should be no more than 770px. I recommend 760px.
*Do add to the width for every padding and spacing you apply.
I went with 925. It turns out that less than 1% of my visitors are at 800x600 or smaller. So I designed the site to display the way I intended to the 99%.
There's a little side scrolling for that handful of people, but it beats squishing everything together for the majority.