Judging by his Amazon page he has clearly sold a large number of books and, even allowing for the probability that he has worked hard to remove negative reviews, he has a lot of positive reviews from readers. The only XJW books with more reviews (he has 403 reviews), as far as I can tell, are Crisis of Conscience by Raymond Franz (692) and Leaving the Witness by Amber Scorah. (483) That’s not bad going. I would call that success, even if there is no accounting for taste on the part of his readership.
As for length, for example try telling Robert Caro how long the typical biography should be and he’ll triple it for you, and them some, and his readers love it. I’m not comparing LE’s work to Robert Caro by the way! I’m just saying I don’t believe in set lengths for genres.