Change Masonry Gallery Image Size

I’m wanting to change the image size of the images in my masonry gallery. I know I have the option to choose between “small” and “medium” when editing in the post, but those are still pretty small. Especially on mobile! I’ve messed with the thumbnail sizes under Media, but that’s not changing anything. What am I missing? Thank you!

The page I need help with: [log in to see the link]