Give an idea of how long your finished manuscript will be. Book length can be stated in number of words or pages. Pages are double-spaced, of course.
If you don’t yet have your manuscript written (and you shouldn’t; you should be writing your nonfiction* proposal first), you will have to make an estimate. For heaven’s sake, make sure your projected length is within the lengths the publisher publishes. That information should be on their writer’s guidelines.
If you’ve already written your three sample chapters, that should help you be able to estimate how long the rest of the chapters will be.
If you haven’t gotten that far yet, you might want to wait until you’ve created your chapter summaries, or at least mapped out how many chapters you will have.
If you’ll have 12 chapters and your prospective publisher wants 60,000 words, can you write an average of 5,000 words for each chapter? Do you have enough material? Do you have too much material?
This is a good average length for a nonfiction book:
12 chapters x 5,000 words = 60,000 words
When estimating number of pages, there are about 250 words on a manuscript (double-spaced) page.
* FICTION WRITERS: If you’re writing fiction, then publishers will want you to finish your manuscript before they make a decision. This is for first-time authors. Established authors may not have to, but you’re not there yet. If you are, why are you reading about how to write a proposal?! I’m sure you already know this stuff. ;-)
If you’ve written your novel, give them the length in number of words and/or pages. If you haven’t completed your manuscript, give them your best estimate.