Change thumbnail size?

The width of the images is always a percentage of the available width. So if you want bigger images, set the number of columns to a lower number and the image size to medium. If you want smaller images, set the number of columns to a higher number and the image size to thumbnail. I use this way because it makes the albums responsive. If you want a fixed width, you can add a new rule to your css:

.epa-image {
    width: 100px !important;
}

Replace the 100px with your desired size.