It is a little too early to make any specific promises, but here’s basically the plan:
Vattu will be 4 books, each in the vicinity of 300 pages but maybe way more. They’ll be published individually in formats to match the first one: 6 by 9 inches with similar design elements and paperback and hardcover versions. Prices will probably be consistent (30 paperback/60 hardcover). The whole thing will be too big to ever fit into one book, so there will never be one book of all of Vattu. Even Order of Tales (~800 pages) is pushing it, and Vattu will be longer.
Waaayyy down the line, when the whole thing is done and I’m phasing out that first format, I might consider doing a 2-volume set, as that would be easier to deal with. It would be nice to have it in two books that are sold together and not prohibitively expensive, and just be done with it. Ooo, maybe two books that come with a matching art/appendix book and they all go in a slipcase together. But I am not in any position to guarantee that that will happen!