In the next two days, I finished the list of jobs, wrote the introduction, and two of the appendices (appendixes? I'll have to look that up)
I also realized that there were a lot of fiddly little differences between page sizes, and they all made a difference. You pick one from a list of size formats the publisher offered, formatted your text for that page size, and moved forward from there. Publishers will resize/format your work for you, for a fee. My word processor software allowed me to select page size, so I figured I'd do that work and save some money.
I'd chosen a likely-looking format when I started: A5. It "looked about right" on the screen. Unfortunately, that's a European format, and I'm publishing in the US, so for the most part, A5 format isn't offered. I needed to find out what was in common use in the business world.
I'd also just recently found that the production cost of my book was likely to be in the range of $6, so I also wanted to find out what business books sold for, so I could guesstimate my profit.
Thus, I returned to Barnes & Noble, this time with a tape measure. More trips up and down the business & self-improvement aisles later, I had found that there were two main paperback book sizes near A5. One was 5.25x8 (inches) and one was 5.5x8.5. It looked like most publishers did one, or the other, but not both. Picking up a sample of books in these formats, I found that the prices ranged (for the most part) from $12.95 to $16.95, a nice tight range.