Recently, I've been warning a lot of people about signatures and I'm getting very tired of doing so. -.- Our rules about signatures are very simple and won't take you five minutes to read. I can forgive a newbie for not knowing what our rules are, but a lot of the people who I've linked to the signature rules thread have been here long enough to know them.
This box here should be the average signature size. A little bigger than that is fine; just that a lot of people are posting signatures that are twice the size or sometimes even bigger than that. Also, signatures shouldn't have too much animation. We're not being big fat meanypantsies for saying that you shouldn't have overly animated signatures; the fact is that some people who are on dialup or have slow connections might find it hard to view the board if attacked by loads of animations. A bit of glitter or a slow animation is fine.
If you find a pic and it's far too big for your signature, then there's a handy little site called iaza that you can head to in order to resize it. Find it here: iaza: Free Online Image Conversion - Simple & Easy Converter (Images, Graphics, Avatars, Anime ...)
You will have to save the pic to an independant image host after resizing, as iaza doesn't save pics for long. Tinypic is a good one. If you need any help with resizing, you can always PM me; I don't bite, honest.