Set Default Image Insert Size in Editor

<?php
// Valid values: 'thumbnail', 'medium', 'large', 'full'.
// Change the filter return value to use a different size.
add_action( 'admin_init', 'scseed_set_default_image_size' );
if ( ! function_exists( 'scseed_set_default_image_size' ) ) {
    function scseed_set_default_image_size() {
        $desired = (string) apply_filters( 'scseed_default_image_size', 'large' );
        if ( get_option( 'image_default_size' ) !== $desired ) {
            update_option( 'image_default_size', $desired );
        }
    }
}