I can only answer the first two questions:
1. what text size are you using on modern LCD screen or eInk?
I format my ebooks so that they display on my ebook reader in the actual size, and currently I use a 13 point base font size. The rest of the text, such as headings, are increased in size relative to that base size.
For readablity, I use the Georgia typeface for serif text and Calibri typeface for san-serif text. Both are very legible on my ereader, and I like the way they look.
2. have you used smaller text size but on prolonged period it had negative impact on your eyes?
I have to wear reading glasses for any type of lengthy reading. I've found that a 13 point font size allows me to comfortably read for long periods of time. I can read smaller text without too much difficulty, but a 13 point font size is the most comfortable for me. I haven't noticed a negative impact of reading smaller text.
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