Also check with local nurseries that carry pond supplies. Many of them will also carry the pond liner in 10' or 20' wide rolls. If you live in an area that freezes, you will want to decide if you plan to shut down the waterfall over the Winter or keep it running. If you want to keep it running all year, you need *depth*. Those pre-formed liners are only 18" deep and will freeze all the way through in many areas, so you want a pond at least 36" deep (this also helps a little with the resonant sound of the waterfall). And how do you want to trim the edge of your pond? Do you want it covered in plants, flagstones, river rocks? In most cases, having a flexible liner is easier to work with.
You asked about the pond shape... with a rubber liner it can be any shape and depth you desire. Consider the picture below, the main oval is about 6' x 14' with the taper leading into a stream at the bottom. When you have an odd shape, you splice together pieces of rubber to fill in where you need. Obviously a bigger pond gives you a lot more to work with. Like you, I specifically wanted a large waterfall just for the sound of the water, and I dug up a large chunk of yard to make it happen. I pump around 5,000 gallons per hour over my falls, which isn't nearly as much as it sounds like. Before you start building your pond, consider looking at samples at local stores or online, compare how large their pumps are, and decide just how much water you actually want to push, THEN create a pond suitable to capture the flow.
And just remember... no matter how big you make your pond, next year you will wish it was bigger.