How to Get Smaller Thumbnails in Categories Display?

Hi there - I am building a site for my sister and LOVE the visual beauty of the Catch Everest Theme. However, one thing I would like is to make the image thumbnails for the categories display much smaller - so that I can see a few posts at a time instead of just one or two and having to scroll down for more. Can someone tell me how to make the images smaller - about 200 x 200 max? Thanks so much (P.S. - if it involves having to build a child theme, I don't know how to do that - if you could point me in the right direction to learn how if that is involved in the solution to my categories thumbnails problem - THANKS!)