Changing the size using post_thumbnail

On my site, I am using the following command to show the featured image on a post:

<?php the_post_thumbnail(); ?>

This is currently showing at 160*155 pixels, but I want to make it smaller than that. I can't figure out how to do that, any ideas?

I have the following in my functions.php file:

add_theme_support( 'post-thumbnails' );

Do I do something on there?

Reading through some of the codex, I just tried the following, but it makes no difference:

functions.php

if ( function_exists( 'add_theme_support' ) ) {
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 150, 150, true ); // default Post Thumbnail dimensions (cropped)

// additional image sizes
// delete the next line if you do not need additional image sizes
add_image_size( 'category-thumb', 100, 9999 ); //300 pixels wide (and unlimited height)
}

home.php
<?php the_post_thumbnail( 'category-thumb' ); ?>

Thanks