When you go to settings -> edit banner, it gives you the maximum dimensions it can be, which is 1650x250 pixels. That will make sure it stretches all the way from one end of the page to the maximum width Weasyl can be expanded. I've seen people have banners that are shorter, and the site just aligns them to the left. If your banner is longer than the browser viewing it, the site automatically resizes the banner so that none of it is cut off, so I went ahead and used the entire allowable length.

As for what you put on it, I would say anything that catches the eye and makes someone pause over your page and realize you have some material there. A part of an art piece or something.