The limits are pretty obvious and clear and if somebody doesn't bother to check or ignores these then I don't see why a member of staff should bother sending them a PM. I see the idea, and how it might be a nice idea, but maybe a better suggestion would be for people to be more careful and make sure their signature does fit the guidelines.

If somebody isn't sure about what the whole pixel dimensions, filesize means or how to check it, maybe they can just pm a Cid's Knight first and ask them to check if it's okay before they put it in. Otherwise there's no reason why they really should have to make it bigger, if they were just being careless, and to me warning them to makes the process longer and more complicated than it needs to be.