|Arthur Conan Doyle's longest Holmes story was 59,452 words
A lot of novel-length Sherlock Holmes pastiches fail at the level of verisimilitude for me before I even open the cover. Why? Because they are too lengthy.
I just picked off of my shelves several popular (and in many ways well done) novels of Sherlock Holmes published in recent years. Each was 300 or more pages. That's in the 100,000 word range. No Canonical Holmes novel was nearly that long.
The April 1960 issue of The Baker Street Journal carried an article by Charles E. Lauterbach called "The World Length of the Adventures of Sherlock Holmes." In it, he lists the number of words in each Holmes short story and novel, from "The Veiled Lodger" at 4,499 words to The Hound of the Baskervilles at 59,452 words.
A Study in Scarlet has 43,483 words, The Sign of Four 43,372 and The Valley of Fear 57,881, according to Lauterbach. Some writers contend that these longer Holmes adventures aren't even long enough to be novels but are more properly considered novellas. I disagree with that. But as there is no real definition of how many words constitutes a novel or a novella, this remains a matter of opinion.
My own mystery novels are in the 60,000 to 65,000 word range, which was popular in the 1950s and 1960s. Many of the Rex Stout and Agatha Christie novels are about this length, for example - around 200 pages, give or take a few.
The longest Holmes short story isn't all that short, by the way. "The Naval Treaty" is a hefty 12,701 words. Stout's shorter Nero Wolfe stories, generally considered novellas, are not much longer at mostly around 20,000 words.