The prototypical laptop has a 15.6″ screen.  When most people picture a laptop, a computer of this size the image that comes to mind.  They a good size for viewing, yet small and light enough to be portable.  They tend to have more features than thinner, lighter laptops, because there is more room in the case.  Whether you want a basic machine, a day to day workhorse, or a fast computer, you will be able to find one that meets your needs with a 15.6″ screen size.