I think that, with an extremely long piece of writing (say, longer than a short story) you'd be best off posting excerpts to the trouble areas.
The short answer is that posting very long pieces of text for critique probably wouldn't work all that well, even if the question met the critique guidelines in all other ways. But read on!
A few words about Stack Exchange sites
Stack Exchange sites, unlike discussion forums or other, less focused Q&A sites, are fairly strict in how we structure questions and answers. Questions need to be of a sort that can be answered definitively and canonically. Answers that the community judges to be good are upvoted, and those answers will "float to the top" of the page, so it's obvious at a glance which answers are best.
Also, questions should be of a sort that will be useful to other people in the future. (Remember this last part.)
Background about critiques on Writers.SE
At first glance, that would appear to disallow critiques, since a critique is more of a freeform activity. However, early on, the Stack Exchange folks realized that a writing site is pretty useless without a critique function, so they set up a very specific set of guidelines. In short, you need to provide context - tell us where the text is from - and you need to ask some specific questions about the writing. "How do I make this better" is disallowed. "Does this achieve [goal]" is good. The community later expanded on these.
Critiques are meant to be focused, meaning that there need to be specific questions asked.
How does this relate to super-long critique questions?
Posting a link to a novel in progress, while possible, probably wouldn't net you all that many responses, for reasons of time. But, putting that aside, it still wouldn't be all that useful to future visitors.
A single scene or short story, posted with specific questions, can be instructive to other writers. A longer work is an entity in of itself, and any lessons learned would be hard to abstract to other pieces of writing without serious study.
Questions also need to be reasonably self-contained. The reason many questions link to the text of the scene/story is to avoid posting text on a site whose terms of service state that any text posted may become Creative Commons licensed, something that can be a problem when trying to sell a book. (The issue isn;t all that clear, so see this page for more information about that.) But putting a book's worth of text outside of a question makes for an even more lopsided situation than with existing critique questions.
All that said, if someone were to try, the mods would keep an eye on the question. There may be a way to do this that would work within the Stack Exchange model.
You can also attend our weekly writing chats, and build relationships there with other users. We have writer and editors who regularly attend, and you may be able to find some beta readers there.