There isn't a way to crack into the forum's code THAT much for it to analyze a person's signature and tell its final size on a per-pixel level AUTOMATICALLY, you know.
Also - it's kind of difficult to decide on a set of rules that constitute what are signatures allowed and not allowed to have. Limiting the amount of pictures in it by rules is not efficient, as images can range from being small inoffensive icons to huge quiz results, redundant 'adoptables', trainer cards, and other pointlessly annoying things. Limiting a signature size to a certain height is also ineffective, because, no offense, but not everyone in the forum has adequate brain power to check the size of their sig in comparison to something.
The closest thing we could do to prevent people from abusing signature sizes is to limit the amount of characters that can go into a sig - as we HAVE done once before - but that just ends up pissing the lot of us off and in the end still doesn't do much but get complaints out of everyone.
So, there's no easy answer I can give you, Mudskipper, and no easy answer I can give you, Shiny. Basically, your profile window (display, rather than edit) shows you a preview of your signature - but because it's in compressed window format it's hard to tell how large it would look like on the forums themselves as parts of posts.
In the end, we must trust the common sense of our members to NOT abuse signatures, and edit signatures that bother us - as we reserve the right to do.
Be thankful, we could have made that a warnable offense.
Also, Shiny: I don't see what relevance does "I know I'm not an admin" have to do with your question - this thread was posted here for a reason - to allow feedback to flow from EVERYONE, not just admins. If we wanted to keep this a discussion closed to admins, we wouldn't have posted it here.