Small is beautiful (for file size) in terms of hard disk storage - but not for high quality reproduction! You should aim to strike a balance between compact file sizes and quality by relating you image to its’ intended use. You can then determine the optimum size. There is no point in having a file size larger than you need.
For example, an image that will be displayed on the Internet should be small (to transfer quickly over phone lines). In fact, it does not need to be any higher resolution than 72 DPI (screen resolution) at its intended size (otherwise it would be wasteful). Also, it does not need to have more than 256 colors - so the GIF format would be the first choice. Such a file would be, ideally, no more than 10KB.
Alternatively, should you require a higher quality image (with more colors but still with minimal size) then a JPEG file would be better.
However, if you know that you want to write to a film recorder then a 6MB (probably TIF ) file would be required. If you want to output to a high-resolution printer at A3 size then a 30MB file would be more appropriate.