You can use the get_post_thumbnail_id to get the ID of the thumbnail, then with the function wp_get_attachment_image_src you can get the image of the size you want, including full size, using the second paramenter.
$thumb_id = get_post_thumbnail_id(get_the_ID());
$thumb_data = wp_get_attachment_image_src( $thumb_id, 'full');
Now in $thumb_data you have a nice array, or false if there’s no image.
The array has this structure:
[0] => url
[1] => width
[2] => height
[3] => boolean: true if $url is a resized image, false if it is the
original.