[setishock]

It can be any size you want it. Height-wise I try to stay around 300px so it looks neat. Width-wise if you have your forum in float mode a good background that blends with the logo works fine. If in fixed width mode just make the logo the same width as the forum.
But either way the height of the header is based on the height of the logo image. If you use just a color as the background it will bloom out to fill anywhere the logo image doesn't cover. If you use a sliver of the logo like I discuss in my tut it has to be the same height or sliced off the logo itself.
It's not complicated really. Here's a tut I threw together based off your question. The image is too big to post here so here's the link to it > http://animeappeal.com/showthread.ph...=7259#post7259