You can force your BuddyPress site members to keep their activity stream updates limited by characters (just like Twitter) by using BP Limit Activity Length plugin.
After activation BP Limit Activity Length plugin adds a character/word countdown next to BuddyPress Post Update button on frontend and it prevents your members from typing further than the limit specified by the admin.
It doesn’t make any changes to old activity feed instead it works only with status updates. The amount of characters is a can be set from admin area settings panel.