Resize WP_Mail Attachments

You can use these filters to customise settings, here the filter names are listed with their defaults.
resize_wp_mail_attachments_max_total_size = 10 (megabytes)
resize_wp_mail_attachments_fit_max_attempts = 5
resize_wp_mail_attachments_fit_reduction_amount = 0.98

For exampple, to set the email size to 25mb add code like this to your functions.php:

    add_filter('resize_wp_mail_attachments_max_total_size', function(){ return 25; });