Hello,
Please check this manual to find out how to add different labels: https://xtemos.com/docs/woodmart/faq-guides/add-custom-labels/
You can change the view of the label by means of this custom CSS:
body .labels-rounded .onsale {
color:#ffffff;
background-color:#000000;
font-size:12px;
}
It is not possible to change the size Sales label only, the size change would be applied to all the labels. Please consider using rectangular labels, you can change the shape in the Theme Settings > Shop > Product labels.
Best Regards