title_body: How does the kindle define how many "book size dots" to display?
upvoted_answer: it is accurate. i found at least 10 sizes for 15 plain text book's (azw3 with no  pictures or complicating formatting), according to there reading time.
  
  "approximation of how long the book is." means: how big is the file. not how many syllables characters or words there are (that's why it says "approximation"). book's with pictures charts or other formatting, or PDF's will have more dots.