WordPress: Draft plugin solution the problem posted here http://wordpress.stackexchange.com/questions/204280/change-the-size-of-the-image-preview-on-the-media-edit-page

wpse-change-image-preview-size.php

<?php
/**
 * Plugin Name: Change the size of the Image preview on the media edit page
 * Author:      Birgir Erlendsson (birgire)
 * Plugin URI:  http://wordpress.stackexchange.com/questions/204280/change-the-size-of-the-image-preview-on-the-media-edit-page
 *
 * ------------------------------------------------------------------------------------
 * Possible workaround, without core modifications,
 * but note that the javascript crop-selection numbers will not be accurate, 
 * when $inf <> 400
 * ------------------------------------------------------------------------------------
 * Puzzle: It's interesting to note that the 'image_resize_dimensions' filter 
 *         doesn't get fired inside the 'wp_ajax_imgedit-preview' action
 * ------------------------------------------------------------------------------------
 */

add_filter( 'admin_memory_limit', function( $limit )
{
	add_filter( 'image_resize_dimensions', function( $value, $orig_w, $orig_h, $dest_w, $dest_h, $crop )
	{
		if( 
		      ! did_action( 'wpse_resize_dimensions' ) 
			&& empty( $_REQUEST['history'] ) 
			&& ! empty( $_GET['action'] ) 
			&& 'imgedit-preview' === $_GET['action'] 
		) {
			do_action( 'wpse_resize_dimensions' );

			$inf = 600;  // <-- Default is 400 - Modify this too your needs!
			$ratio = wpse_image_get_preview_ratio( $orig_w, $orig_h, $inf );
			$dest_w = max ( 1, $orig_w * $ratio );
			$dest_h = max ( 1, $orig_h * $ratio );

			return image_resize_dimensions( $orig_w, $orig_h, $dest_w, $dest_h, $crop );
		}
		return $value;
	}, 10, 6 );
	return $limit;
} );

/**
 * Our modified version of _image_get_preview_ratio()
 */
function wpse_image_get_preview_ratio($w, $h, $inf = 400 ) 
{
        $max = max( $w, $h );
        return $max > $inf ? ($inf / $max) : 1;
}