These are all great questions.
When I was younger and took the train, I did most of my reading during commute with paperback books that were smaller in size and would fit in the inner pocket of my coat. Larger pages make for less creasing of the binding but end up keeping a hand occupied or needing carried in a satchel or backpack.
Most hardback physical books I acquire in recent years are timeless data for reference lookups (math formulas, languages, science, medical before the purge and rewrite of reality)
Today though most of my reading is in electronic medium like web, pdf or epub. Audiobook format tended to prefer for longer works without diagrams.
I find it hard to listen to an audiobook that references diagrams or where the material really relies on the diagrams to convey what's being discussed. If you listen to some of the pieces read by Guy Swann you can see what I mean.
If your new book has this dependency on diagrams, consider a bundle option to be able to offer both the ebook and audio at a discount.
In all likelihood, I'd personally probably start with the ebook format and if I hadn't read it after a time may get the audio version.
