I don’t know if there’s a rule of thumb for the correct size. There are so many factors involved and some of them are very subjective. I think what you did is a good practice: checking books similar to yours and see the size they use. Regarding the discrepancy in size between word and Indesign, I haven’t seen that. I usually export a few pages to PDF and then print it to have a good idea of how it will look.
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