Changing featured image thumbnail

I have a theme that allows me to select a featured image which should have the dimensions of 900×300 px the problem is that on the blog preview the thumb is 150×150 and doesn’t always look good. How can I add the ability to load a custom thumbnail for this area? Using a custom field? Currently my theme calls the thumb like this:

<?php the_post_thumbnail('blogsmall', $attr );?>