The most important point here is that a document should put across a well-defined concept. It is not generally worth splitting one idea arbitrarily into two bits in order to make the bits smaller. Nor is it a good idea to put together ideas which are really separate just to make a bigger document.
A document can be as small as a footnote .
There are two upper limits on a document's size. One is that long documents will take longer to transfer , and so a reader will not be able to simply jump to it and back as fast as he or she can think. This depends a lot on the link speed of course.
The other limit is the difficulty for a reader to scroll through large documents. Readers with character based terminals don't generally read more than a few screens. They often only absorb what is on the first screen, as if that is not interesting they won't be bothered to scroll down. Readers are also put off by being left at the top of a large document.
Readers with graphic interfaces generally scroll through long documents with a scroll bar. When the scroll bar is moved a small amount, the document should move a sufficiently small amount so that some of the original window-full is still left in the window. This allows the reader to scan the document. If the document is any bigger, then it is basically unreadable, in that any movement of the scroll bar will loses the place and leaves the reader disoriented.
Advantages with longer documents are that it is easier for readers with scroll bars to read through in an uninterrupted flow, if that is how the document is written.
Also, one doesn't have to go to the trouble of making (or generating) so many links and keeping them up to date if things are altered. If making the links is a problem, just settle for one link to a contents page. Some browsers have "next" and "previous" buttons to allow a document to be browsed serially according to a list.
(In fact, one can normally scroll up and down explicitly page by page, but this is gives the same feeling as the terminal interface.)
A rough guide, then, for the size of a document is: