WordPress creates 3 different sizes as far as my research goes. Read this on WP image handling -> http://www.studiograsshopper.ch/web-development/wordpress-featured-images-add_image_size-resizing-and-cropping-demo/
You can easily customise the default Media sizes by changing the default sizes in Dashboard > Settings > Media.
You can grab any of those sizes. See the codex @ http://codex.wordpress.org/Function_Reference/get_the_post_thumbnail
The function you want is:
get_the_post_thumbnail( $post_id, $size, $attr )
And here’s an example:
get_the_post_thumbnail($post_id); // without parameter -> Thumbnail
get_the_post_thumbnail($post_id, 'thumbnail'); // Thumbnail
get_the_post_thumbnail($post_id, 'medium'); // Medium resolution
get_the_post_thumbnail($post_id, 'large'); // Large resolution
get_the_post_thumbnail($post_id, 'full'); // Original resolution
get_the_post_thumbnail($post_id, array(100,100) ); // Other resolutions