Help with finding sytlesheet for single-product image

HI, I’m sure this is a theme problem, but of course because it’s the weekend, any questions I have posted for the developers are unanswered.
I have used chrome developer and inspected the element of this page:
https://www.plumdigitaldesign.com/product/chainsaw-massacre/
(yes my clients own a gun store..)
Anyway, I have a featured product image, and then an image gallery within the add product page.
The theme I have installed has a thumbnail gallery and a slider, but the problem is, no matter what I do, I cannot get my slider images to be 600 x 450. I can see the element in the chrome developer, and play with it, and adjust it to look the way I would like (right now it’s set to 605 x 454), but I cannot find anywhere in the css that I can change this to my desired size of 600×450.
I just thought someone out there might know how to find the width and height element I am seeking. I am truly a novice, and I thought someone else could help me!
Thanks, Heidi