You can set a page size in Word. Don't use 8.5 x 11, use whatever the real, final page size will be. Also set the correct margins. If you're self-publishing and producing your own master, you can produce the PDF or whatever format directly from this and you'll know exactly what will be on every page. But even in general, it will at least give you a rough idea of how much fits on each page and how many pages the final book will be.

I did my second book with the intended final page size and margins in Word and I found that helpful to give me a feel for how long chapters would look, as well as to help me keep toward my targetted total length. In my case I figured that 250-300 pages was a good length for that particular book and I had it divided into five sections so I wanted at least roughly 50 pages each. So as I was working I had a feel for whether I was beeing too wordy or not wordy enough, whether I needed more material or had to trim, etc.