You didn't have to type them all out; I know what the options are.
Picture Uploading Permissions:Signature Image Maximum Width (pixels)
Signature Image Maximum Height (pixels)
Signature Image Maximum Filesize (bytes)
Those restrictions only apply to images that are uploaded; not those that are inserted with the IMG code.
If you are keen on limiting sizes, then do what I did and disallow IMG tags in signature images and force users to upload them. This way, your size restrictions cannot be bypassed.