You determine the size of your IXXI yourself, there is no limit to the dimensions. However, it is important to consider the correct resolution when ordering a large-sized IXXI. You can read more about this here.
IXXI cards are available in two sizes: 10x10cm and 20x20cm. These sizes cannot be combined in a single IXXI. This is due to various factors including the cutting margins of the cards, the available IXXI tools for clicking the cards together and the distance between the cards.