Okey i think i understand what you want to do.
Here is a function that changes the html-output of the feature image in admin. It will look for the attached feature image ID and output it by using wp_get_attachemt_link() so you can change the size by the parameter $size. Here is a function that should work:
function wpse_111428_change_feature_image_admin( $content )
{
global $post;
$size = 100;
$id = get_post_meta( $post->ID, '_thumbnail_id', true );
if( $id )
{
return wp_get_attachment_link( $id, array( $size, $size ) );
}
}
add_filter( 'admin_post_thumbnail_html', 'wpse_111428_change_feature_image_admin' );
I found the original code in \wp-admin\includes\post.php
that WordPress uses to output the feature image.