Hello,
a) Banner element does not have custom class option. You can add custom class for the column and use it to apply styles for the banner inside this column.
b) Edit banner element and remove the min size for the banner.
c) If you use zoom effect on hover then it won’t have border-radius because it’s image border-radius, not the banner block border-radius. So, you may try to add border radius for the banner element using custom code
.banner {
border-radius: 20px;
}
Regards