A great rule of thumb in determining how big you can blow up an image file is to use the standard print resolution of 300 dots per inch (dpi). If your camera is taking photos at 2400x3000 pixels, divide that by 300 and you'll realize you have an image that can accurately be reproduced to an 8x10 print. From about.com, here's a basic chart for smaller photos (which you can do the math to expand upon):
- 4x6 1200x1800 pixels 2 megapixels
- 5x7 1500x2100 pixels 3.1 megapixels
- 8x10 2400x3000 pixels 7.2 megapixels
So, obviously, the most important thing to consider is when taking your photos you need to have your camera settings adjusted to capture an image which can be printed at your desired size. There's no way to take an image that was shot with enough pixels for a wallet sized photo look good when blown up to a poster size.
Once you've decided on the image you want to enlarge, you basically have two options for printing. You can go to a commercial printer that does wide format printing and have them create your desired size for a commercial price or you can do it yourself. Now of course you're saying, "I don't have a printer that can spit out something that size!" Depending on your finished product, you don't need to. There are services online such as Block Posters that will chop up an image and give you downloadable PDF files that you can then print on your own printer and reassemble into one large image. Or, you can forgo any paid services and try this technique (http://www.cnet.com/how-to/print-your-own-giant-posters/) using the Rasterbator photo tool (Windows version available here - http://arje.net/rasterbator).