BuddyPress.org

Actually I think it is impossible without modifying the core files. Any character limit or other custom validation is lost in the edit profile page, and since the $error is a local variable on xprofile_screen_edit_profile, there is no way to set it. Can you confirm that @djpaul?
I don’t know if this is planned for future releases, but I could help build a better validation code for user registration and edit profile. Adding a character limit to fields would be nice.