How to include featured image on blog post preview?

You can edit the default image sizes that WordPress creates by going to Settings -> Media in your WordPress dashboard and specifying image sizes. However, this will change the default image sizes and will affect all images across your site.

Another option is to edit the template file that is displaying your blog (likely home.php or index.php) using get_the_post_thumbnail()

You can use get_the_post_thumbnail() to insert the post’s featured image while specifying a size.

For example: <?php get_the_post_thumbnail( get_the_ID(), 'thumbnail' ); ?>

get_the_post_thumbnail accepts the following image sizes: post-thumbnail, thumbnail, small, medium, and large.

Links to the docs:

get_the_post_thumbnail();

get_the_ID();