How to change thumbnail size

Add this code to functions.php (use custom child theme or plugin FunctionsPHP):

function peach_set_thumbnail_size( $thumbnail_size ) {

	$thumbnail_size = 'full';
	return $thumbnail_size;

}
add_filter( 'basic_singular_thumbnail_size', 'peach_set_thumbnail_size', 10 );

Change full in second string:

• ‘thumbnail’ – Thumbnail (default 150px x 150px max)
• ‘medium’ – Medium resolution (default 300px x 300px max)
• ‘large’ – Large resolution (default 640px x 640px max)
• ‘full’ – Original image resolution (unmodified)