To answer you questions:
1) How large do your documents get?
Depends on the game, we are currently working on a 2D sidescroller game so it will probably not be 1000's of pages.
But the dream project would be huge! (we have written a RPG storyline and only the dialogue and basic screen play are close to 1000 pages) A game designer should expect that he will need to write many pages before going into production and not just wing it. You would need a really tight knit team to wing it and work with people that know exactly what you mean all the time.
But the bigger the team is the more detailed the GDD needs to be so that they don't have to run to the main designer with 1000's of questions all day long. I can update you on how large the 2D GDD ends up being at a later time.
2) Is every team member expected to read the whole thing?
Every team member will get the GDD but they only need to read the part associated with their position on the team. I would prefer them to read the whole thing to get into the spirit of the game but is not a demand.
3) Is it living? And if it does change, is everyone expected to re-read it?
The GDD will be so welled planned that it will never change. it is there to keep development focused and so that everyone knows
what needs to be done and what the end result is supposed to be.
Ideas can change along the way if something doesn't work out but the GDD stays the same. You can write update pages which you can hand out if necessary. The MAIN GDD does not need to be modified or re written during or when the game is complete, that would be a waste of time when you are in the middle of production. Essentially the GDD together with the art documents are like the blueprints or technical drawings (how something functions or has to be constructed)
I'm not saying that how I do it is the law because the reason this is even a question is because there are no clear rules, how a GDD looks and what is in it is up to the Game Designer in the end. But I feel that having a solid foundation will be crucial when you are in development.
We don't have a dev team or the capital to back one up so when the doc is finished we got the back bone to hopefully start getting a dedicated team together. It all depends on the individuals and the community if the game will ever get made but if it ever does then I would love to be able to share the GDD with the community.