"Has anyone found a limit to CAE in terms of the size/complexity of models/database size in Mb?"
This depends on your machine resources.
"Is there a point where CAE refuses to continue and decides the best option is to crash/quit?"
Yes, but again it depends on your PC.
A while ago I tried to use Abaqus/Viewer to post process a model of nearly 2 million nodes on my Win2K machine with 2GB of memory, it started off okay, but when I tried to change load case it gave up ! I then moved to a colleague's XP machine with 3GB of memory and had no troubles.