how to re-size Thumbnails width & height in theme functions?

i recently added a position to post title with a transparent background so it stays at the bottom of post thumbnails. everything works fine with thumbnails that have bigger pixels because the theme resizes them back through set_post_thumbnail_size( 604, 270, true );
the problem is with thumbnails that have small pictures, i mean the theme does not force all pictures to take 604, 270, it’s just a max- size.
how can i force all thumbnails to take these values?
—
HTML :

<div class=”entry-thumbnail”>
<?php the_post_thumbnail(); ?>
</div>

PHP (functions.php) :

set_post_thumbnail_size( 604, 270, true );

& finally, my website if that would help solving out my issue : elegance-style.com
p.s i am using fruitful theme.