change a single porfolio project image size 7.1

Site URL: https://www.franklinprefab.com/cottage

When I change portfolio image size of one portfolio page, it changes all the other portfolio pages as well. How do I change the image ratio of just one portfolio page? I am using the Grid: Simple layout.

Squarespace's website states: These changes affect all Portfolio Pages using the same layout.

I feel like I need to add code, but can't find where I can copy/paste this action. I am using Version 7.1.