add_image_size not working

For this to work, you must have added the thumbnail support. You can use the following code in functions.php to add the support

add_theme_support( ‘post-thumbnails’ );
add_image_size('poster-full', 450, 9999, false);

and to display the image:

if ( has_post_thumbnail() ) {
 the_post_thumbnail("poster-full");
}else{
 echo "No thumbnail";
}