First, it's important to understand a few terms that relate to image size and resolution--PPI, DPI< and Megapixels. If you're not familiar with these terms, or you need a refresher, follow the links below for a more detailed explanation:
Pixels per inch (ppi) - A measurement of image resolution that defines the size an image will print. The higher the ppi value, the better quality print you will get--but only up to a point. 300ppi is generally considered the point of diminishing returns when it comes to ink jet printing of digital photos.
Dots per inch (dpi) - A measurement of printer resolution that defines how many dots of ink are placed on the page when the image is printed. Today's photo-quality ink jet printers have dpi resolution in the thousands (1200 to 4800 dpi) and will give you acceptable quality photo prints of images with 140-200 ppi resolution, and high quality prints of images with 200-300 ppi resolution.
Megapixels (MP) - One million pixels, though this number is often rounded when describing digital camera resolution.
When determining how many pixels you need, it all boils down to how you will be using the photo and what size it will be printed. Here's a handy chart to guide you when determining how many pixels you will need for printing standard size photos on an ink jet printer or through an online printing service.
5 MP = 2592 x 1944 pixels
High Quality: 10 x 13 inches
Acceptable Quality: 13 x 19 inches
4 MP = 2272 x 1704 pixels
High Quality: 9 x 12 inches
Acceptable Quality: 12 x 16 inches
3 MP = 2048 x 1536 pixels
High Quality: 8 x 10 inches
Acceptable Quality: 10 x 13 inches
2 MP = 1600 x 1200 pixels
High Quality: 4 x 6 inches, 5 x 7 inches
Acceptable Quality: 8 x 10 inches
Less than 2 MP
Only suitable for on-screen viewing or wallet-size prints. See: How many pixels do I need for sharing photos online?
Greater than 5 megapixels
When you get beyond five megapixels, chances are you are a professional photographer using high-end equipment, and you should already have a handle on the concepts of image size and resolution.
Digital camera manufacturers would like all customers to believe that higher megapixels is always better, but as you can see from the chart above, unless you have a large format ink jet printer, anything over 3 megapixels is more than most people will ever need.
However, there are times when higher megapixels can come in handy. Higher megapixels can give amateur photographers the freedom to crop more aggressively when they can't get as close to a subject as they would like. But the trade-off to higher megapixels is larger files that will require more space in your camera memory and more disk storage space on your computer. I feel the cost of additional storage is more than worthwhile, especially for those times when you capture that priceless photo and may want to print it in a large format for framing. Remember, you can always use an online printing service if your printer can't handle large format.