The number of books included in the Crime Writers' Association (British) and Mystery Writers of America's lists of the top mystery novels of all time is daunting, totaling 149 entries.
If you want to tally up your own been-there, read-that list, you could start with A Woman in White, the earliest entry, published in 1860. It also happens to be the longest, at 810 pages and 260506 words.
Instead, in its place, you could just as quickly read six shorter novels: The Third Man, Double Indemnity, The Postman Always Rings Twice, The Thirty-Nine Steps, The Daughter of Time, and Devil in a Blue Dress. (You might be able to squeeze Little Caesar in there -- 160 pages, I couldn't find its word count.)
Here are the novels on the CWA/MWA list which are fewer than 65,000 words. Included are those that rank #1, 2 and 3 as the best all-time mystery novels, namely, The Daughter of Time, The Spy Who Came in From the Cold, and The Big Sleep (all available as ebooks for a quick download). I can vouch for these: they make for great reads.