Change Featured Image Size of the Post

In you WordPress Admin, Go to Settings > Media, here you can set the default size of ‘thumbnail’,’medium’ and ‘large’.

If you read through the codex page of the_post_thumbnail() you would have found that it has parameters that can be used to fetch a precised size of the image.

the_post_thumbnail();                  // without parameter -> 'post-thumbnail'

the_post_thumbnail( 'thumbnail' );       // Thumbnail (default 150px x 150px max)
the_post_thumbnail( 'medium' );          // Medium resolution (default 300px x 300px max)
the_post_thumbnail( 'large' );           // Large resolution (default 640px x 640px max)
the_post_thumbnail( 'full' );            // Full resolution (original size uploaded)

the_post_thumbnail( array(100, 100) );  // Other resolutions

So, you can try the_post_thumbnail( array(100, 100) );