Most people actually juice fast, as opposed to water fast. Water fasting you can (this is my impression) only keep up for a few days before it becomes counter productive, juice fasting however, you can keep up for a month or more.
Fasting will reduce your stomach size, this is logic.
I haven't yet done my first fast, but I'm very keen on doing it.
You will lose weight, I'm under the impression that alot of that weight will be toxins leaving your body, which would mean better skin, less cellulite, etc...
With juice fasting, you don't necessarily 'make up' the calories, you can just assure that you consume enough, say 1,200, to prevent your metabolism lowering.
I have heard alot of merits about both types of fasting, I have also heard alot of people who go against plain water fasting, arguing that you starve your body of nutrients, making it harder for your body to cleanse itself.
I would suggest you do the research, and research what SEEMS to be good for your health, don't just do it for weight loss, or you may get addicted to the wrong thing, i.e. starvation.
Good luck, it will be tough to start, but i've heard that after the first two days, it gets alot easier.