Signatures must follow all regular content rules, but they also have some specific restrictions on size.

1. Signatures may have at most 512 visible characters, and 1024 characters in total. This allows you to include BB or HTML code outside of the character limit.
2. Signature images have a maximum total vertical image size of 200 pixels, a maximum total horizontal image size of 700 pixels, and a maximum total file size of 80KB.

The "total vertical image size" is the total vertical space used by images, according to positioning. For example, you can have two 150 pixel high images beside each other in your signature (total vertical image size of 150 pixels), but they couldn't be on separate lines (total vertical image size of 300 pixels, which exceeds the limit). The total horizontal image size is the same idea, just horizontal instead of vertical.

Signature images have never really had specific restrictions (before these rules were created), but very large images have never really been allowed. These restrictions should not cause problems for most people, and are more just a way of setting objective guidelines for image sizes.

Signature character restrictions should not be a problem for most people either. Remember that you can have up to 1024 characters, but only 512 can be visible. This will allow you to use formatting tags outside of the character limit. These limits are checked when you edit your signature (the admins aren't going to be counting the characters manually :-P).

Administrators will usually ask you to change your signature if it falls outside these restrictions, though they have the right to edit it immediately.