You can set the length of the hashtag by navigating through:
Dashboard >> WB Plugins >> Hashtags >> General
You will have two options here:
Minimum hashtag length – Allows you to set minimum hashtag length. The default value is 3.
Maximum hashtag length – Allows you to set the maximum length of the hashtag. The default value is 16.
Note: For setting length at widgets you have the option inside
Dashboard >> Appearance >> widgets >> BuddyPress Hashtag/bbPress Hashtag >> Limit.