Hi, I had a similar question, this is how i managed the full-size image question :
<?php $thumb_id = get_post_thumbnail_id( $post->ID );?>
<?php $image = wp_get_attachment_image_src( $thumb_id,'full' ); ?>
<?php echo $image[0]; ?>
In the first step i get the thumbnail Id
The second allows me to get the image src info (cf. wp_get_attachment_image_src) in full size.
the last echoes the first value returned wich is the url.
