No such feature or module exists to moderate the pixel size of signatures. Anyone with a clearly disruptive or offensive signature will be dealt with by a global moderator.
The moderator action line is part of moderator policy. It is not always adhered to, but meh, I have the moderation logs to know if anyone's been abusing the old power. As for more transparency, whenever something significant happens I always make a post about it.
I think there ought to be a limit on
number of images - as that's easily (and objectively) measurable at first glance. In my opinion, one image is a suitable limit for this purpose (lots of images in a signature, particularly animated GIFs, tends to distract while reading posts around said images - it also eats up my [tiny] download quota when mobile, as an aside).
I proposed a size limit (with in file size and dimensions), but obviously these will need to be enforced manually due to technical constraints as you mention. Such enforcement is prone to 'turning a blind eye', but as long as a reasonable size (i.e., 400x300?) is set, reasonable moderator discretion will be sufficient.