Re: where is the water stored??
Find where, on the inside of your hull, a plastic tube meets that "gas fill" looking thing. Follow the tube, and you'll eventually locate where it goes into your potable water tank. The tank is probably mounted low, and may even be below decks. If it's permanently mounted without a hatch, you might even have to cut a hole to see it.
However, mostly they're just hidden instead of actually foamed in place.
An older water tank may have some dirt, debris, scale, or other unhealthy things in it, so I'd suggest you flush it out at a minimum with some cleaner once a year - there are products for this that iboats sells.
If it's a really old potable water tank you may wish to replace it. The tanks are sold on iboats, too.
Be aware that if you have onboard potable water you probably also have an onboard blackwater tank - IE a sewage tank. It may not hold the output from your head (toilet) but it probably will be where your sink drains to - it's illegal in most cases to just drain it overboard. This tank will look a lot like your other water tank, but definitely isn't drinkable water.
As far as size, the capacity is usually molded into the tank or printed on an attached label.
Erik
