If you’ve finished your novel and are on to figuring out how long your query letter should be, I believe I have a scientific answer!
Using the data from the “Successful Queries” column , I have found that successful queries are 239 words on average with a standard deviation of around 91 words (based on 41 examples).
While there’s more to success than word count, the winners of the query battle seem to be between 240 and 420 words.