Laptop/desktop

When using a laptop/desktop computer, you have the option of choosing a card format upfront -  that is, before choosing a design. This option can be found beneath each card design on the page on which you see all of the designs. You cannot change the format after designing the card.  In this case, you'd have to recreate the design.

Tablet

When using a tablet, the option of choosing a card-format can be selected right after choosing a design - you can swipe through the available formats of the chosen card.  You cannot change the format after designing the card.  In this case, you'd have to recreate the design.

Smartphone

When using a smartphone,  you have the option of choosing a card-format upfront -  that is, before choosing a design.  This option can be found in the 'swipe through' menu beneath each item on the page which shows all of the designs for a given category.  You cannot change the format after designing the card.  In this case, you'd have to recreate the design.