media sizes, thumbnail sizes, featured image sizes

Hi,

I’ve been having a hard time figuring out how to change thumbnail sizes in my Twenty Thirteen child theme from 604 x 270 to the size I want.

I semi resolved the issue by going into functions.php of Twenty Thirteen and changing the value of : set_post_thumbnail_size( 604, 270, true ); to what I wanted.

I would rather leave Twenty Thirteen untouched, but if I add the “thumbnail_size” line to the functions.php in my child theme it doesn’t seem to register the new size, it always goes back to 604 x 270.