title_body: Big screen dimensions or high DPI for vision impaired users? I'ld like to get my dad an ebook reader for Christmas. Yet, he is a bit vision impaired which means that he only has a good reading experience with bigger and sharper/clearer/more-acurate font rendering. 
  
  To be sure the display renders the fonts as clear as possible at a zoomed (125%) font-size, I would like to know what I should focus on… should I focus on a big screensize (display dimensions, width x height) or should I rather go for a high DPI screen? What advantage would the one have over the other (from a vision-impaired reader's point of view)?
upvoted_answer: I would suggest going with an iPad with Retina Display not because:
  
  
  Its a bigger screen
  At the current time, better resolution than a few other devices
  
  
  BUT because Apple requires their typeset to be adjustable and coded in to accommodate for visually impaired and not a fixed font size.  So what this means is if your dad buys a book on iTunes, he tries to adjust the font size and fails, he can request a refund on the book and Apple will require the publisher to correct the issue to re-activate their content to be sold.
  
  My experience with Amazon is across the board and is generally a "How many users have complained base" and their support is across the board.  If he does have an issue with the book he can get a refund but the smaller device would be harder for your dad to read perhaps. 
  
  This may be an extension on your question but the quality of the content is as much important as the device you are getting.