By postcard format do you mean crop the picture into a postcard shape?
If so, here is one way: Activate the Rectangular Marque tool. Set Style to Fixed Aspect Ratio, and enter the dimensions for Width & Height. In this example I used a standard postcard size of 6" x 4.25". Open your picture, draw out the rectangle, and Image > Crop. The picture will now be in the 6 x 4.25 proportion.
To get it to the actual 6" x 4.25" size, do Image > Resize > Image Size. Check the Constrain Proportions Box, and enter either the true Width or Height; the other dimension will change accordingly.
If this is not what you are trying to do, please be more specific.