I find the narrowest margins on my Kindles fine. Possibly because I read exclusively on Kindles for many years. I don't use the narrowest possible margins on my Kobo either.
Ditto for line-spacing.
As to font sizes, I use the font_ramp hack on my Kindles. Sometimes I switch between sizes 4 and 5 on my custom scale. On my Kobo I seldom switch between font sizes. On my Nook I use customized base font size and so I never switch there.
I should add that all my ebooks have uniform margins, line-spacing and base font size, so I don't have to change them between books.
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