These days we don't have any set limit so long as it isn't obnoxiously huge. Users are able to hide the signature of any other user if they wish by selecting "Hide this user's signature" when they hover the mouse over the bottom left corner of the user's signature they wish to hide. Therefore bigger signatures aren't as big of a problem as they used to be so we don't need to have such strict enforcement on it anymore.
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.
If you would like to know some recommended sizes that people generally use for making signatures, a very common one is 400x150. I personally use 450x170. ~420x~160 is another one that is used. That general aspect ratio seems to be preferred to have enough room for a subject and any other focal points that may exist in an image background. I would suggest not going wider than 600px and taller than 200px but there's no actual rule against it; it would just be your personal preference for workspace.