[Theme: Pashmina] Thumbnail is too big!

Some thumbnails of my blog posts take up almost the entire screen, while others don’t.

If you look on this page,
http://comfymommy.com/category/product-reviews/
the first thumbnail is extremely large. How do I make it smaller so that it still shows the full image, and with the post title?

I have little to no experience with code. I appreciate any help. Thank you!