No answer from anyone, so I guess my comments about there being no recommended standards is confirmed!
What we choose to do was:
* Make the large, high quality PDF, available from our repository (which has primarily an academic audience)
* Make a smaller, lower quality PDF, available from our exhibition software (which is focused at both academics and the general public), with a link to the high quality PDF in the repository
Initially, we were thinking of a 20MB file size limitation on PDFs in our exhibition software; but ended up increasing that to around 40-50MB. Our rationale there was that because we had embedded the BookReader software, and the BookReader software downloads on a page-by-page basis, users would be able to check items in the BookReader before choosing to download the PDF. So if users did choose to download the PDF, it would be because they knew it was what they wanted, and thus they'd be prepared to put up with a larger download.