Invitation cards come in rectangular and square shapes. When it comes to square cars, the smallest of them has a side length of 5.5 inches, the medium ones are 6.25 inches, and the large ones have sides of 6.75 inches. Rectangle cars are more common. They also come in a wider variety of sizes. The more affordable ones are the smallest rectangle cards that come in A2 size. That means the longer side is 5.5 inches and the shorter side is 4.25 inches. For larger invitation cards, folding cards are better as they will be more convenient to send.