I certainly can't say for sure but my guess would be that they would not.
My theory is that a lot of the reason behind the current guidlines are to keep the size of a page / post within the screen at 800 x 600 resolution. A lot of people still use that screen resolution and would not like scrolling left and right to see an entire post. I'm sure that the clipart in signatures could be somewhat bigger than the current guidelines and still maintain that but you know as well as I do that there is already clipart out there that "stretches" the guidelines and if the size limits were raised, some would stretch them even further and make the pages too big. I've already seen some people that don't like how long some of the signatures are getting as they don't like having to scroll so much to see the next reply.
I'd guess that the file size limit has more to do with the load time for the pages. So many people are on dial up connections that if the guidelines were expanded, it would take even longer for each page to load. I know that a lot of the dial up folks already have images and signatures turned off.
This is just my own opinion though.