Change size and crop medium_large images

You can call add_image_size() again to update the existing image size. Assuming it is called medium_large (Events Manager Pro is a paid plugin so not very much people have it) you can do something like:

<?php
function update_medium_large_size_wpse216595() {
    add_image_size( 'medium_large', 600, 255, array( 'center', 'top' ) );
}
add_action( 'init', 'update_medium_large_size_wpse216595', 11 );

Then you will need to regenerate the thumbnails. Although I am not 100% sure that init is the right hook for the job.

p.s. You can check this if you are using a child theme.

p.s. 2. I am not sure that I understand the center cropping correctly. You many need to play with it a bit. array( 'center', 'top' ) will crop the picture in the center (‘trimming’ the right and left part).