Resizable Featured Image?

Hi, I’m using the Gavick Pro Perfetta theme which I love but was wondering how can I resize the Featured Image? It’s so HUGE and I would perfer at the most be 500×700. Currently, I removed the picture I was using as to set Featured Image. I searched online but am not finding what I need. It could just be I’m not sure exactly what to find.

Thanks in advance! 🙂