Always start with the Codex. According to the function reference for
(string/array) (Optional) Image size. Either a string keyword (thumbnail, medium, large, full), or any custom size keyword defined by add_image_size(), or a 2-item array representing width and height in pixels, e.g. array(32,32).
Default: 'post-thumbnail', which theme sets using set_post_thumbnail_size.
PLEASE NOTE: The crop does not work in Wp 3.0+. All that is needed for WP 3.0+ is the call for the thumbnail to post. Then proceed to media in the dashboard and set your thumbnail to crop to the size you wish to use.
You need to create your custom image size, via
add_image_size( $name, $w, $h, $crop ), and then call
the_post_thumbnail( $name ).