Instead of setting a size, you can make the information of size dissapear when you choose Full Size. It´s ridiculous when you say you want the full size image and they try to resize it.
Go to /wp-includes/media.php on line 84 and comment that line (//)
// any other type: use the real image and constrain it
// list( $width, $height ) = image_constrain_size_for_editor( $meta['width'], $meta['height'], $size );
This way instead of
<img src=”…” class=”…” width=”…” height=”…” />
you will get
<img src=”…” class=”…” />
=)
