This thread shows how to prevent images in signatures.
I assume you mean the image dimensions, and not the file size?
Allowing images but limiting the dimensions would be trickier.
One way would be to customize the do_markup and undo_markup functions in ubbt.inc.php to handle width and height attributes. That would be a potential maintenance headache, since you'd have to deal with any changes made to the markup code when doing version upgrades.
Something else you could do would be to use the getimagesize function to check the image dimensions when a signature is updated, and reject the signature if the image is too large. This would have the limitation that someone could add a link to a small image, and then later change the image file itself to a larger image, without changing the link, and you'd have no way to detect that.
