From what I understand, you just want to have a thumb created at 120 x 120 when an image is uploaded using the new post type. Then use that image as the featured image on the custom post pages.
Just add a new image size
add_image_size( 'companies_thumb', 120, 120, true);
Then in the post type template for companies you just call the thumb you defined.
<?php the_post_thumbnail('companies_thumb'); ?>