Fairy Theme Featured Images

I’m curious what the best image size is for fairy theme. I’ve used the square image size in the past, and currently I’m using 400w/600h, but my site speed is also really slow, so I’m doing everything I can to optimize. If that means changing my featured images, I’d love to know now so I can change if necessary. Thanks for your help!

The page I need help with: [log in to see the link]