Recently I read the O'Reilly Google Hacks book. It suggests a successful web site should have at least 100 content pages, with each page of 5 to 15kbytes, with a minimum of images. It occurred to me that such a web site is about the same size as the average book, divided up into three printed page sections. Perhaps this is not a coincidence and represents a useful unit of information for people to use.
If you have 100 web pages each of 10kbytes that is 1,000 kbytes. Assuming one quarter of the web page is formatting, that leaves 750 kbytes of text. I did a quick survey of a couple of books within reach and found they were about 300 pages long with about 2500 characters per page, or about 750 kbytes of text in total. That doesn't sound very scientific, so I did a search and found a report from University of California saying the average book has 300 pages and .75 Mbytes of plain text equivalent.
That the average book is the same as the minimum size for a web site should not be a surprise. Both books and web pages are designed to be read by people and so people set the limit on size (just as the size of a business card is not set by printing technology but by the size of a human hand).
Previously I have done web based books with one web page per chapter. This was easy to maintain, but a whole chapter on one web page seemed a bit long for the reader. I got some complaints from readers and more recently noticed Google AdWords had difficulty working out what these chapters were about so it could put relevant ads on them.
Following Google Hacks' advice, the amount of information in the average web site should be divided into 100 pieces, each the equivalent of 3 printed pages. My quick survey found about ten chapters in the average book, so each chapter would be made up of ten sections, for a total of 30 pages per chapter.