With a smartphone/iPod-type device, the HTML formats will show up as a smaller text. As long as their font sizes aren't hardcoded into the pages, you either make sure your browser settings allow for dynamic resizing, or you edit the HTML files so that they look larger.
Not sure about the other formats. It might depend on the app you're using to read them. I use the OverDrive Console app to read eBooks from my local library for free, & it allows for a few different text sizes (not to mention a nice "white text on black background" option for readability).
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