Thanks for the replies, guys. I had already tried stripping out the bric-a-brac (which is how you spell it, btw) but that did not significantly reduce the file size. I did just copy the whole floor over to a new plan, and I was able to make the changes to the ceiling height w/o any problem there. I can use that file for the current purpose (trying to see how the crown molding will die around a corner and how far into the entry to carry a soffit, questions that the contractor asked me yesterday since he's about to build it), but I'm still unclear about how the file got that big, why it won't get smaller, and why setting the default ceiling height at the beginning should matter. In this case, the clients went back and forth multiple times about how high they wanted the ceiling in the kitchen to be (and we ended up w/ two different heights), so I have no idea how many times I changed it. I am a kitchen and bath designer, and I frankly didn't pay a whole lot of attention to the ceiling heights in the other rooms on the floor since I wasn't working on them. I'm not a newbie, but I don't see why, if I need to change all of the ceiling heights in the course of working on the project, it should cause this problem. Is it more likely to be related to the file size?
One thing that I did do at one point many moons ago was import the architect's drawing as a picture to confirm that my measurements matched hers. I'm pretty sure I deleted it, and I can't find it in the plan, unless it's still hiding back there somewhere. (I had set the brightness to 100% to leave it in the background in case I needed it again.) Could that have done something??
It's been a looooong project w/ these clients, so I'm not sure what else I might have done to make the file so unwieldy, but I'd be interested in hearing any other thoughts.