Maximum dimensions are 400x150 pixels with a file size limit of 70kb.
Site rules:
https://forums.d2jsp.org/info.php?p=9Quote
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.