When creating your backgrounds you first want to start with a sketch that you can place in your Photoshop file. When you open a new project on Photoshop, or what ever program you may be using be sure that you have the right size. CS5 has a default film and video setting so getting one started is simple but if the program you are using does not have a film setting the format you can use is below.
Size: NTSC DV
Width: 720 pixels
Height: 480 pixels
Resolution: 150 pixels/inch (you can also use 72 but that is very small and hard to get good details)
Color: RBG/ 8 bit
The pictures below are the backgrounds I used for the animation I am working on. the edges are not finished in these pictures. When you are doing your animations be sure you finish you edges. Even though your most important information will be in the center, you will want to be sure the whole background looks clean.