How to Adjust Size of Portfolio Pictures

I am an artist and currently using the Highend theme. I have the Portfolio- Simple theme set for my photography page. However, it looks like the page is adjusting the size of the thumbnails to fit the page. I would like all the thumbnails to be the same size (to look as smaller icons) and when you click on the photograph, for it to open in a new window to original size. I’ve tried multiple things to adjust this and can’t seem to find the answer. Could you please help? I would like for the page to look similar to Etsy in the attached file.