Check this:
http://forums.d2jsp.org/info.php?p=9 
Quote
Signature Rules and Guidelines (violations may result in loss of rights to have a signature):
The entire size of your signature shall not exceed 400 pixels wide by 150 pixels high.
Text sections of your signature shall not exceed 6 lines of text if you include no images, and 3 lines of text if you have an image.
Total size of the images contained in your signature shall not exceed 70 kilobytes.
Your current signature is fine.
Avatars will automatically size down to the correct dimensions, however they must not exceed 50kbs.
If you're asking what the best size to make a signature is, then honestly any size that is less than 400 x 150 is fine. With that said, that maximum is commonly the standard that is used here. Sometimes you'll see ones around 300 x 150, 400 x 100, etc.
If you have any further questions about this, feel free to contact me
This post was edited by Fault on Aug 5 2012 05:48pm