When you create a new document, Photoshop offers you a pop-up list (the Preset option) of the most commonly used document sizes in terms of width, height, resolution and colour mode. This includes sizes such as A4, Letter and the more popular web page sizes. However, you will often find that you use other sizes over and over. So rather than type in the details each time, you can just enter the size and resolution you need, give it a name and then save it as a preset using the Save Preset dialog box.
For example, I sell t-shirts on CafePress. The size required for T-shirt designs is 10 inches by 10 inches and requires a resolution of 200 ppi. So I saved this as a preset called T-shirts and now it appears at the top of the list. You can make as many custom document sizes as you want, so save yourself some time and make some custom document sizes.