This is a question I think a lot of people have. I’ve been asked a lot and I have a hard time answering because I don’t count the number of words in the queries I read. I don’t even have a good sense of how much room it should take up on the screen because that varies widely based on font and such. I just know that some queries feel too long, some too short, and some just right.
Recently, though, I took a class where we were learning statistics. For my end of semester projects, I analyzed sample of 140 query letters (34 which resulted in requests and 106 which received form rejections from our agency).
I found that the average word count for the successful query letters was 357 and the average word count for unsuccessful letters was 407. The results were different enough to suspect that the differences are statistically significant.*
Furthermore, the standard variation for the rejected letters was much higher than for the successful letters. In simple terms, this means that the successful letters were very consistent in their length. All but two were within 75 words of the average. On the other hand, the rejected letters varied drastically in length. A couple were over a 1,000 words long, while several were barely 100 words.
What this all goes to show is that there is a fairly standard word length for queries. Based on my studies, I would suggest trying to keep your queries between 275 and 375 words in length.
*The p-value was actually .06, which is slightly above the .05 standard for significance. However, it’s close enough that I’m comfortable assuming the data is significant.