Or, to change the actual size of the thumbnail, you need to copy the alx_setup function in functions.php over to a child theme (sample child theme avail in theme options help dropdown) and modify the thumb-large/thumb-medium sizes as you wish.
I tried your suggestion and the featured image on the front page of the blog remained the same size (345 width by 550 height, strangely enough). The original image size is 150×240.
Perhaps I don’t have the relevant section. This is the code I attempted to change so all width-height ratios were 160, 100:
/* Theme setup
/* ------------------------------------ */
if ( ! function_exists( 'alx_setup' ) ) {
function alx_setup() {
// Enable automatic feed links
add_theme_support( 'automatic-feed-links' );
// Enable featured image
add_theme_support( 'post-thumbnails' );
// Enable post format support
add_theme_support( 'post-formats', array( 'audio', 'aside', 'chat', 'gallery', 'image', 'link', 'quote', 'status', 'video' ) );
// Declare WooCommerce support
add_theme_support( 'woocommerce' );
// Thumbnail sizes
add_image_size( 'thumb-small', 160, 100, true );
add_image_size( 'thumb-medium', 160, 100, true );
add_image_size( 'thumb-large', 160, 100, true );
// Custom menu areas
register_nav_menus( array(
'topbar' => 'Topbar',
'header' => 'Header',
'footer' => 'Footer',
) );
}
}
add_action( 'after_setup_theme', 'alx_setup' );