You can choose the card size and format before editing and personalizing it.
Please note that it is not possible to transfer an already edited card to a bigger size or format. In this case, you'd have to recreate the design.
Here is how you can choose the format:
From laptop or desktop
This option can be found beneath each card design on the page on which you see all of the designs.
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.
From the App
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.