How to change max height of container?

Default max height of BP Better Messages container for desktops is set to 500px.
There is bp_better_messages_max_height filter which you can use to change that value, this is example of how to use it:

add_filter('bp_better_messages_max_height', function($max_height){
     return 600; // New max height in PX
});

Perfect place to put this snippet is child theme functions.php file.