If your theme is using the_post_thumbnail()
to display the featured image, adding the following code in functions.php
file will display the Thumbnail size of the image (as set on Settings -> Media Settings):
add_filter('post_thumbnail_size', 'my_thumbnail_size');
function my_thumbnail_size() {
return 'thumbnail';
}
Or you can display the featured image in any width (X) or height (Y) you wish:
add_filter('post_thumbnail_size', 'my_thumbnail_size');
function my_thumbnail_size() {
return array(X,Y);
}