crofty is correct in the size. The width of the forum post itself is 728px and the 100px is so that signatures remain signatures rather than just as long as posts. When you go to edit your signature it gives the rules right above the box to edit.
Text from the signature area
Your signature may contain:
• Up to 1 images
• Images up to 728 x 100 pixels
• Up to 1 URLs
• Up to 2 lines
Text from the signature area
Your signature may contain:
• Up to 1 images
• Images up to 728 x 100 pixels
• Up to 1 URLs
• Up to 2 lines