If you enable post thumbnails for your theme, you will find that you may not like the default size. If you want all of your post thumbnail images to be the same size throughout your theme, then you can set it in functions.php:
set_post_thumbnail_size( 100, 100 ); // 100 pixels wide by 100 pixels tall, box resize mode
OR:
set_post_thumbnail_size( 100, 100, true ); // 100 pixels wide by 100 pixels tall, hard crop mode
