Then try to define
your interpretation of the word "book length" and you have your answer.
Hint: Byte, characters, words, sections, chapters are all valid here as they are reasonable fixed entities. Not so for "pages" as they tend to depend on several variables like line-spacing, font-size, screen-size to name a few.
Further, it also depends on what you want to do with that info. To determine the virtual
thickness of the e-book in the sense that "big" means a lot of reading pleasure (or not
)?
Personally, I'm never interested in book lengths: it's the book contents whats make me decide whether I will read it from front to back, or just some snippets.
Just my 2cts.