Change thumbnail size for Media Library

This was an interesting question as I knew nothing about modifying the default behaviour of the Media Library. So, I ended up doing some research on the matter.

I found out that the Media Library uses wp_ajax_query_attachments() to get the images and other media. Inside the ajax function there’s ajax_query_attachments_args that can be used to modify the arguments for the attachment WP_Query.

After the WP_Query the ajax function uses wp_prepare_attachment_for_js (function and filter) on the queried posts before they are sent back with wp_send_json_success().

Perhaps you could try playing around with wp_prepare_attachment_for_js filter and see, if you can achieve the result you want.