Thumbnail Sizes

Hi,

I have setup a wordpress blog using a theme which supports featured image.

When the posts are listed the thumbnail is the correct size. Nevertheless when I go to view the posts as a single post the thumbnail stays small – but I want it to display the full width of the post – how would I do this?

I had a look at function.php and made a new add image size:

add_image_size( 'single_post', 640, 200, true );

And I changed get the post thumbnail in single.php to:

$image_url = get_the_post_thumbnail($post->ID, 'single_post', array('class' => 'postThumb', 'title' => the_title_attribute('echo=0')));

But its still not making the thumbnail go full width when viewing a single post.

Could anyone point me in the right direction?

Thanks,
Kyle