Sample sizes are: 4x6, 4x9, 5x7, 5.5x8.5, 6x9. There are other sizes available. Post cards tend to be 4 color on the front, and vary on the back. They can be 4 color on the back, or just 1 color. With or without a bleed. Many are on 10 pt or 12 pt card stock with a coating on one side or both.