in my experience, GE can handle a decent amount of stuff.
I tried doing an overlay with a background image. Admittedly it ran a bit slow (was trynig to overlay a large bmp) but otherwise, it worked.
Additionally, I tried overlaying a picture of the earth over the ENTIRE earth. It functionally worked, didn't look nice....but it worked.
Just try it and see what happens!
oh, and I don't think there's a limit to kml's...if you look at the statue of liberty model, when you unzip that and look at the underlying kml of it, it's 9.8 megs!!!!!!!
The computer will limit it more than GE (it slows down when it tries to render 10 meg models a bit :-P )