There are two ways to reduce the size of the scan - first you can reduce the size. This means the width/height by either cropping the image or reducing the size of the image.

Second you can also reduce the size by lowering the quality of the image. JPG images allow you to compress them when they are saved. The software you use to manipulate the image should allow you to do this. Of course when you increase the compression (or JPG calls it lowering the quality) it will start to look washed out/blurry as you get too low. For the most part 60% will not be too noticeable.