Digital stamps can be resized before printing so the same image can work on different card sizes.
Keep the proportions locked
When resizing, keep the width and height linked so the artwork does not stretch or squash.
Start from your card size
Decide how much space you want the image to occupy on the finished card, then reduce or enlarge the stamp to fit that area.
Use the right file
PNG files are useful when layering artwork digitally. JPG files are simple to place on a white page. Where an Easy Print PDF is included, use it when you would rather avoid manual resizing.
Print a test first
If you are unsure, print on ordinary paper before using your preferred card stock.