BuddyPress.org

You can edit the functions.php file of your theme to change the length of the excerpt.
Make sure to backup functions.php before doing anything. And do the changes in a test server first.

This is the code I just tested on twentytwelve and it does the job (can be refined though).

Just copy paste the code below into functions.php and change 9999 with the excerpt length you prefer

function firewire_longer_excerpts( $excerpt_length ) {
    $excerpt_length = 9999;
    return $excerpt_length;
}
add_filter( 'bp_activity_excerpt_length', 'firewire_longer_excerpts' );