Size limit?
 

I'm creating a potentially huge Outline for a research project ... dumping entire scans of books into it ... and I'm wondering if there's any effective size limit on an OO outline. File size, or word count, or number of items, ... anything?

There's no limits enforced so it will really be up to the Mac you're using. Depending on the size of your scans, I would probably recommend linking them in instead of embedding them just to be on the safe side though.