What is an 'average' file size for Revit?

There are many variables to consider with respect to file size - complexity of the design, size of the building, number of levels, source of the content loaded, end-user methodologies, and certainly L.O.D. To answer your question, any file size is acceptable as long as it still performs well. In my 7+ years with Revit, I've never had a project hit 350 Mb (at least not after I cleaned out all the unnecessary junk people put in it). You may want to check out Autodesk's Model Performance Technical Notes and possibly AU session AB100-1P (Managing Large Projects in Revit Architecture).

i find about150mb is average for a 40k sf bldg... you should purge unused families to lower file size

Anywhere between 200mb-1gb, I would say.

I don't really think there's an 'average' size, you should be fine with almost any size unless there's something wrong with your computer.

I do a lot of unique stuff that really is far beyond what Revit was intended to do. It's not unusual for my project files to exceed 1000MB. Because things move extremely slowly in projects that size, I do most of my work in the family editor where the components are much smaller and less complicated. Then, I assemble them in the project. I get a lot of crashes but I realize I'm pushing the limitations constantly. I know when to anticipate a crash usually so I do a save if I suspect I'm going to be in "dangerous" territory. Consequently, I haven't ever lost anything of any significance due to a crash. I always cancel the crash reports to Autodesk. It would get to be a little bit redundant trying to explain repeatedly why I am working with project files that are 1.5GB. I have 64 GB of RAM. I suppose if I upgraded to 128, I would get fewer crashes but it's not practical to work with files that big because manipulating them is like watching grass grow. Slow and boring.

Honestly, I don't pay attention to file size. Always have lots of memory (16GB on a 64 bit machine) and terabyte disc drives and you should be OK. Hardware like that is pretty common this days. Keep your site info as a link file, and all engineering/consultant files as links and you should be OK. I have worked on ultra high rise towers in Revit and havehad no problem.
One more thing: don't over-model. You don't need to show the bolts on your steel structure. You'll probably get it wrong anyway.

Around 400mb is fine, mine get up that high.