Mmmh, 少女終末旅行 doesn’t have (almost) any furigana, so there is no too small font here I think there was an instance where a word was written in kanji and had a furigana in katakana spelling a word in english that I didn’t bother reading. Other than that, I didn’t have any problem reading the balloons.
Now that you make me think about it, I will have probably your same problem with some manga on the kindle (I read aria and nonobyori with a magnifying glass where my poor eyesight couldn’t do). Obviously you can zoom with a kindle, but it depends what resolution the pages are, if it’s too low it will get just blurry.
An alternative might be the use of an ipad/tablet. You’d be still bottle necked on the file resolution, but the experience would certainly more fluid.
I’ll try to take a couple of pictures tomorrow to try to give you an idea (it’s 3am here now… I need to zzzzz).