1. Images can only be a maximum of 125px high. Width doesn't really matter, but keep in mind that most people only have monitors that go to 1024x768 (or they don't know how to change the resolution, in which case I pity them). Therefore, making your signature more then 500px wide is generally considered rude and will get you a nasty PM from one of the moderators.
2. If you DO have a 125px image, please only include one or two lines of text as well. I don't want to see a 125x400px image followed by Moby Dick in it's entirety, or I'll wave my magical banhammer and you'll be gone.
3. DO NOT post advertisements or links to advertisement sites, "Free PS3" or "Free iPod"-style sites, or any other pyramid scheme or scam. Do not link to retailers. The only exception is users with Sponsor accounts who would like to post a banner or link to their website.
4. Keep all signature content WORK SAFE.
Remember kiddies, the idea here is to make it so you can still have that bitchin' signature, but so that it won't get in the way of other people who are trying to browse the forums. I'm sure you'll also agree with me when you see someone who responds to a post with one word, but then their signature takes up the rest of your screen. Don't be a retard.
These are pretty well regulated already as this site has a lot of specific guidelines that have to be met before you can even upload the avatar. However, I must tell you the following:
1. Any animated avatars that flash a bunch of annoying colors (especially very quickly) will be changed to a picture with rude commentary about your mother. Or just deleted. Repeated infractions will get you banned.
2. Off-site linked avatars should be no larger than 100x100 pixels. The file size should be less than 30k.
3. Keep the contents of your avatars WORK SAFE.
4. Avatars should not link to or advertise for retailers, unless they are on an AP Sponsor account. This applies to the 'Location' field as well.
Note: This post is subject to change without notice.