Featured image size in column

by

In functions.php :
Make sure featured images are enabled

add_theme_support( 'post-thumbnails' );

then add 

add_image_size( 'thumbnail-news', '100', '75', true );

And in the code above change 

echo get_the_post_thumbnail($post_id, 'thumbnail');

to

echo get_the_post_thumbnail($post_id, 'thumbnail-news');