How big is an image file ?

When the term "size" is used to describe a digital image, there are several different meanings.

Image Size in bytes

This refers to the size of the file in bytes when the image is stored to disk. This is a precise quantity, and one that accounts for many factors. It is quite often the quantity that a magazine would ask for when requesting an image for publication. Note that 1KB = 1,024 bytes, 1MB = 1,048,576 bytes. One floppy is 1.44MB, one CD is 700MB.

In general terms:

File Size = (Image Width x Height x Colour Depth) / Compression Ratio

So an image with lots of pixels (Width x Height) can give you a big file, but if you really (JPEG) compress it (high Compression Ratio) the file can end up small.

A web page thumbnail might be 3KB, saved as a low/medium quality JPEG.
A web page gallery image might be 45KB, saved as a medium/high quality JPEG.
An image you print in a document as 4"x3" might be 300KB, saved as a medium/high quality JPEG.
A high quality magazine submission image might be 50MB, saved as a TIFF file (no lossy compression).

Image Size in pixels

This is a precise quantity. If a digital camera is labelled "4.3 Megapixels", this is referring to the number of pixels in the image. On a web page, this directly affects the size on the screen, as the computer has a fixed number of pixels across and down on the display (for a given choice of resolution).

Small image (45x30) pixels
Medium image (450x300) pixels

Image Size in inches

This is a more abstract quantity. The file may include information about its "resolution" in "dots per inch (dpi)". This may refer to the original size if from a scan, but generally has no direct physical meaning. When you print a document containing this image, the size on the page can be altered by stretching the image or printing at 500% for example, and so this concept of size is not very useful.

A magazine publisher might want 300dpi across a full page (maybe 2550 pixels on the short side minimum).

Image quality

Note that a camera may have a poor lens and interpolate the size of image higher than the actual resolution of the image sensor, and so lots of megapixels does not mean "high quality".