Truncate latest activity in buddypress

You can filter 'bp_get_activity_latest_update_excerpt'. Sample code, not tested, you will need my function utf8_truncate():

add_filter( 'bp_get_activity_latest_update_excerpt', 'wpse_56860_trim_buddypress_excerpt' );

function wpse_56860_trim_buddypress_excerpt( $excerpt )
{
    // Number of characters to show. Adjust this to your needs.
    $length = 80;

    // Fix BuddyPress’ wrong quotes.
    return _x( '“', 'opening curly double quote' ) 
        . utf8_truncate( $excerpt, $length )
        . _x( '”', 'closing curly double quote' );
}

Download as complete plugin.

Update

There is an alternative: create a hovercard.php in your theme, the plugin will use this file then instead of the same named file in the plugin directory. In hovercard.php change the line bp_member_latest_update( 'length=10' ) to get the proper length or add the filter at the top of the file and remove it after the last endif;:

<?php if ( bp_has_members( "include={$_POST['userid']}&max=1") ) : 
add_filter( 'bp_get_activity_latest_update_excerpt', 'wpse_56860_trim_buddypress_excerpt', 8 );
?>
// lots of stuff
<?php endif; 

remove_filter( 'bp_get_activity_latest_update_excerpt', 'wpse_56860_trim_buddypress_excerpt', 8 );