Concerning the size of a chapter, there used to be a limit (or at least an expectation) that an EPUB chapter should be smaller than 300k. See this thread:
https://www.mobileread.com/forums/sh...d.php?t=104370
Very long chapters will, in any case, be a problem for EPUB reader apps, because each chapter is an HTML file, and the reader app will have to load that chapter into memory (and, in the case of some apps, load it into a web-browser control). That does cause significant performance problems (web-browsers do quite badly at displaying web-pages that are 100+ pages long - and EPUB readers will have the same problem).
Also, an EPUB file containing many chapters (say 1,000+) will cause performance problems for most reader apps, because the file is a ZIP archive, and doing a search-by-file-name through ZIP archives becomes a performance bottleneck (time taken grows in proportion to the number of entries in the ZIP file).
Bottom line: a paper book containing 10,000 pages is quite cumbersome. But so is an EPUB book of that size. Maybe someone should add bit to the EPUB standard to let it represent 'anthologies' - i.e. collections of semi-detached books, which are kept together, but do not have to be displayed as one piece of connected text.