How to set post thumbnail size?

First add the following code to your theme functions.php:

add_image_size( 'square-150', 150, 150, true )

In sarkpress-function.php file, change this line:

<?php the_post_thumbnail(); ?>

to:

<?php the_post_thumbnail('square-150'); ?>

You would have to regenerate your images by using Regenerate Thumbnails plugin

Preferences:

• https://codex.wordpress.org/Function_Reference/the_post_thumbnail
• https://codex.wordpress.org/Function_Reference/add_image_size