What is the recommended upper limit on notebook size?
There is no fixed limit on notebook size. Very large notebooks are almost always a result of including large files such as movies or long voice memos as internal attachments copied into the notebook. These attachments have no affect on performance, other than when saving or backing up over a network, or when emailing the notebook.
Data for each page is saved in a separate file within the notebook file package. Very large pages with many entries can exhibit performance problems when resizing the page or opening it for the first time. Notebooks with a lot of text can also cause the index to grow, which can affect notebook size.
It is important for large notebooks to make sure that the index is periodically compacted, by adjusting the "Compact Index" settings in Preferences, or by periodically invoking the Compact Index menu item in the Tools menu.
For best practices, it is recommended that you use the Save To command periodically for the purposes of creating archives (compressed/self-contained).