Well, to be honest, most members have way larger signature banners... No one will mind if you have a banner that is higher than 400p. Just don't add thousands of huge, glittery neon banners. If you have a banner the same size as most members, then that is okay.
Umm.... not entirely true. 400px is huge and most members are below it. I'm not as OCD about it as Rich was, but we do keep an eye on it. If someone has an outrageously huge signature we get them to shrink it down.
The rules say for your graphic to be no higher than 400 pixels? How tall is that?
For instance, ShiningBright's banner is 271px tall. If you're using Internet Explorer to view the forum, if you right click on the banner (or any image) at the bottom of the menu that pops up select Properties. That will bring up a window that will tell you how big the image is. In general, when you hit 400px you'll know because it'll be huge. Just keep it reasonable and you'll be fine.
aislin