How long should a book proposal review be? What are the expectations, primarily in terms of length but also along other relevant dimensions, for a review of a book proposal? The book itself will be as long as 5-10 papers but the proposal itself usually only includes a sample chapter and some overview material. What will editors at academic presses expect back from proposal reviewers?
The editor of the press should have given you guidelines -- or even a response form to fill out. If they didn't, it'd be simple enough to e-mail the editor back and ask him/her how detailed of a response is required.

I personally vary the response length depending on how promising the book proposal is. If the proposal has a lot of merit but is undercooked in some areas, I may spend a considerable amount of time discussing what needs to be done. If the book is stellar and ready to leap onto the presses, then not much needs to be said. 

If the book is terrible because of severe methodological or analytical flaws, my review might be less than a page. If the ms is in severe need of proofreading or editing, I may respond with a just a paragraph noting this and suggesting that I'll re-read when the ms is actually ready to be reviewed.