How can I change the thumbnail size (featured image) in the Directory?

How can I change the thumbnail size (featured image) in the Directory?

When a user browses the categories, I would like the listing to display a much larger thumbnail.

I have browsed through these forums to try and find a solution, but have been unsuccessful thus far. Someone suggested some CSS, but that just stretched the tiny thumbnail and looked terrible.

Here is a link to an example: http://everythinggeraldton.com.au/listings-category/health-hair-beauty/day-spa/

I need the image to be larger when browsing the categories, not just when viewing the individual listing.

Thanks in advance.