Editing font size and hover effect on products page

#1. You can add this to Website > Website Tools (under Not Linked) > Custom CSS (or changing H2 in Site Styles)

.grid-title {
    font-size: 14px !important;
}

#2. Add this CSS code

img.grid-item-image.grid-image-cover {
    opacity: 1 !important;
}
img.grid-item-image.grid-item-additional-image {
    opacity: 0 !important;
}
img.grid-item-image.grid-image-hover {
    opacity: 0 !important;
}

#3. Use this CSS code or change Heading 1 in Site Styles

h1.ProductItem-details-title {
    font-size: 18px !important;
}