I've found that the average size of an image shot at 5 megapixels is between 2-2 1/2 megabytes. I always shoot jpeg fine (least compression). TIFF and RAW files can be considerably larger.
The number and brightness of colors in a scene can increase or decrease file size. For example, if you shoot a scene in color then the same one in black and white, the b&w will be smaller.
When editing and saving a file (always work on copies of originals, btw), the new file can be much larger than the original. It is important to watch file size when you want to email an image or upload to a website.
Related reading:
Image file sizehttp://www.digicamhelp.com/taking-pictures/image_quality.htmPreparing images for web presentationhttp://www.digicamhelp.com/learn/image-editing/image-resizing-for-web.htm
