Here are the most commonly used standard card sizes at Little Harbor Press. All dimensions shown are in inches.
A1 - 3.5" x 4.875"
The A1 size is most often used in fold-over style for thank you notes and in flat style for wedding response cards. If you like to write shorter thank you notes this is the right size for you, but if you tend to write a lot, you may want to opt for the larger A2 size.
A2 - 4.25" x 5.5"
The A2 size is the most commonly used size for a wide range of cards from invitations to announcements. As a fold-over style note card, the A2 gives you a little more room to write than the A1.
A6 - 4.5" x 6.25"
The A6 size is a little less common than its smaller or larger brothers, but it's a great in-between size for flat cards.
A7 - 5" x 7"
The A7 is a larger and more formal size for your most important events like holiday cards and wedding invitations. It has a substantial feel and gives you the most area for large photographs.
Biz (Business) - 2" x 3.5"
This is the size of a standard business card, and is used for calling cards and wedding table placecards.
Square - 5.5" x 5.5"
The square card is another large, substantial card and has a unique look all its own. Be aware that extra postage is required by the USPS for square envelopes!