What are you going to put in the pond? Koi? Goldfish? Are you going to drain it over the winter? I live in Canada so you know what my winters are like. I bubble the ponds to keep an air hole open that allows the pond to breathe and release toxins(decaying matter). In the summer I use a swimming pool filter for a continuous flow between the ponds and I have it set up for a couple of water falls as well. If I knew how to post a pic here I would. Hope this helps.
Make sure that you have 2' extra for the outside edge it will settle. Also take into consideration the type of soil you are putting the liner over, if it is rocky you may need to put some felt under the liner to protect it from becoming punctured. So take the Length add 4' and add the depth x2 as well.
Do the same for the Width,