Currently, We don't have any such rules as we haven't had too much of an issue with large sigs. The site also has the ability for users to hide others signatures from their own sight, so that they can hide sigs that they don't wish to see. Sigs are automatically hidden when users browse on a mobile browser to help with page load speeds.
A good rule of thumb is probably to keep image height under 500px, which is probably already very generous. Just take a look at what others have done and you'll get a good idea of what is typically done.