The Wild Card cartridge is one of my favorite cartridges. I couldn't wait to get started playing around with it. Then I made the mistake of creating a card without an envelope first. So then I went through each envelope to determine the max size for a 12x12 inch mat and a 12x24 inch mat. As you can see, changing the mat size can change the size of the envelope a lot or not at all. The max size indicates the size of the card that will fit in that envelope.