Work in the specs of your acim bookstore. Somewhere in the proposal, note when you can deliver the book, the length, and the format (hardcover or paperback) if that is very important to you and both are options. Publishers generally want the book delivered within six to nine months, maximum, unless you’re writing a work requiring substantial research.
As for length, you can specify page count based on the number of words that fit on a typical page (take a look at a book that is about the size, shape, and length of the book you envision, count the words on a page and the number of pages, then do the math). As an in-house editor, I was taught it’s best to specify word count in contracts so that’s what I do in book proposals as it’s much more accurate than a page count. A typical length for a self-help book these days is about 70,000 words (it used to be 100,000 back in the day), but how many words fit on a given page will vary based on the design of the book.
Include endorsements if you can, and even the promise of a foreword by someone with an impressive name if you can procure a commitment. You might be surprised by who will agree to write an endorsement or foreword at an early stage of the book! And if they say no, you can always come back to them later when you have more material to show them (but don’t promise what you can’t deliver; wait until you get the thumbs up from your foreword writer before including it in a proposal).
Before submitting your proposal, consider having it evaluated by an industry professional who can help you tweak it, fill in any gaps, and make it as strong as possible, whether that professional is a literary agent or a freelance editor with extensive book publishing experience who can help you polish your proposal before it is sent to the agent.
Spell check it, have another set of eyes proofread it (this is where a friend with great grammar skills is a terrific asset), and double check that you have included all the important elements. Make sure your book proposal makes it impossible for a literary agent, and then an editor, to say no to taking on your nonfiction book project!