Each doubling of the ISO number indicates a doubling in film speed so each of these films is twice as fast as the next fastest. Image sensors are also rated using equivalent ISO numbers. To get more light you need a longer exposure time that can lead to blurred images or a wider aperture that gives you less depth of field. All things being equal, it's better to get an image sensor with a higher ISO because it will enhance freezing motion and shooting in low light. Some cameras have more than one ISO rating.
Some cameras even increase the gain automatically.
Dim light requires a fast lens and a high ISO or you have to resort to flash. The size of an image file depends in part on the resolution of the image. The higher the resolution, the more pixels there are to store so the larger the image file becomes. This is a useful feature because there is a trade-off between compression and image quality. Less compression gives you better images so you can make larger prints, but you can't store as many images.