I think it just depends on what kind of things you want happening at the top of your website. Some designers like clocks, weather magnets, Ad banners, RSS feeds and the like. That will play into the size of the actual banner.
My feeling is most people use at least 1024x768 screen resolution and I try to design for that. Certainly, there are those who use 800x600 or use a real high resolution (keep up two browsers or a browser and some other app) but that's not the norm. So, I figure 900px-950px wide is about right. I like to allow the banner to float as the resolution increases. You can fill (or tile) with a pattern or gradient to allow the banner to float and fill as the browser size changes.
You also have the option to use several smaller graphic images and fill (or tile) in between with patterns or gradients. For example, you could use a small logo graphic on the top left and simply fill to the right with your pattern or gradient. The banner on Gimp Chat centers and fills on the left and right with
this graphic. That allows me to extend the gradient and the blue bar on each side of the banner. Some people get creative and fancy with those kinds of things.
If you're designing completely from scratch, you can just dispense with a banner, design your page as a single image and cut it up (as needed) for navigation. You see some really creatively designed sites that use that method, but it can be time consuming.
This site is based on phpBB3 so a floating banner seemed like enough. Oregonian and I designed the other graphic elements (buttons, folders and stuff) on this site and chose the color scheme we liked. We thought lighter color backgrounds on the posts would be best for a graphically oriented website like this one. We used lots of shades of blue.