USEARCH manual > options > -wordlength
-wordlength
 
The ‑wordlength option is an index parameter that sets the length of the k-mers that are indexed. In other words, it is the k in k‑mer.

The word (or pattern) length is a critical parameter that can have a significant impact on sensitivity and speed. The default settings work well for most applications, but for very large datasets and other resource-intensive tasks, it can be worthwhile to experiment with different values. You're welcome to ask me for advice.