We cannot transfer your edited design (your photos and/or text) to another size or format of the same design. In this case, you need to choose the format/size that you would like to receive and create the cards again.
You will find the formats available for a particular design listed when you choose a design (on pc/laptop, under the thumbnail; on mobile/tablet when you tap on a design, then swipe left/right)
Also, note that if you have already placed an order with the wrong size, you can cancel the order before midnight on the same day and the money will be automatically refunded. To do this, go to My Account.