Picking a random word with an specific number of letters in bash

Hello Im just starting to learn bash, and i Need pick a random word with n letters, n will be the parameter.

I have this:

#!bin/bash
shuf -n1 /usr/share/dict/words

So I know how to pick a random word but not with a specific number of letters. If someone can help me I would be very grateful.Thnks!