There is no way to crop wordpress medium images to exact size without editing the soure code. If you want to crop wordpress medium images to exact size as specified under Settings -> Miscellaneous -> Medium Size then open wp-includes/media.php in your favourite editor.
Search for line 191:
$newimage = imagecreatetruecolor( $dst_w, $dst_h);
Change $newimage variable to:
$newimage = imagecreatetruecolor( $max_w, $max_h);
Voila, we’re done!