Greeting card design, and specifications?

Using Adobe Photoshop CS3 design greeting cards, do not know how to use the format, network cards found
is 27.4 * 21.4 (CM)
letter card is 16.6 * 23.6 (CM), but because of the reasons why the postage required The only part of the 16.6 * 11.8
puka is 15 * 10.4 (CM)

the specifications do not fully? Reality of greeting cards so much? I understand this is unreasonable, for cm Unit should be the size of the real world.I could not find time for a greeting card to the volume.

Seek Therefore, the specification is the number? Me that the specifications of this means that I design a greeting card, designed in Adobe Photoshop CS3 Greeting cards, create a new document, the height and width in general should be how many? (Marked attention to the unit)