Change display area for featured images?

Hello,

I really like the theme, it is really something fresh among the themes available at www.ads-software.com. But I would like to have featured images that are longer in px and being box-resized. I had a look in the functions.php and tried to do a little changes as described in the link: https://www.studiograsshopper.ch/web-development/wordpress-featured-images-add_image_size-resizing-and-cropping-demo/

Changed the following:

// This feature enables post-thumbnail support for a theme
		add_theme_support( 'post-thumbnails', array( 'post', 'page' ) );
			add_image_size( 'page-thumbnail', 600, 355, true );

to

// This feature enables post-thumbnail support for a theme
		add_theme_support('post-thumbnails');

But the featured images are still shown in the same area. Is there something in the theme specific code that limits the area available for featured images. If I have to change something else to get the same amount of text under the featured image after doing changes to allow a bigger area for featured images then that would be good to be pointed out as well.

I am not really sure how accomplish that i.e. a 300×250, 300×300, 350×300 or 400×400 image selected as featured images are shown as they are when selected as featured images. I am hoping for some help.

Any help much appreciated!