The default thumbnail size for the Wordpress theme I use generates category thumbnails for the homepage and archive page at 200x110px. I would like to add a custom thumbnail size for a specific category of 200x200px that would display on the homepage and on the category archive page. I have experience working with Wordpress but I am not an expert. Based on the responses I've read to similar questions, I believe the answer lies in functions.php but I need detailed instructions on how to achieve this. If anyone could help, I would greatly appreciate it.