Changing the Featured Image Size in Directory + Fatal Error

Hi, I was trying to change the featured image size in directory and after coming across a couple answers in the forum, I edited the listings php file where it said"the_post_thumbnail( array(50,50)" to "the_post_thumbnail( array(300,300)"

I then got an error message when I click the listing:

And the photo didn't show up at all, so I changed it back to the 50,50 but it didn't seem to fix it. Do you know what I did wrong or how to fix it? 1. so it doesn't give the error and 2. so I get the thumbnail bigger?