Limit the length of your BuddyPress activity updates like Twitter
BP Limit Activity Length is a simple plugin that forces users to keep the length of their activity updates limited by amount of characters or words.
The plugin adds a character/word countdown next to the Post Update button, and prevents users from typing further than the specified limit. Only new activities will be truncated to the set amount of characters or words
The amount of characters is a simple setting in the BuddyPress administration screens.