I have a large backyard goldfish pond. There are great pond forums out there that are worth lurking on for advice. Pond keeping is a lot like chicken keeping in that:
- up front planning makes things a lot easier and less traumatic
- there are always trade-offs and you have to decide what is acceptable to you
- there are lots of different goals in pond keeping and the type of pond that you keep/the advice that applies will depend on these goals
- climate plays a huge factor
Hard plastic liners are good as are rubber pond liners - you have more flexibility with the rubber liners, but rigid liners can be simpler.
If you want fish, plan for filtration, cleaning, water changes, winterization, predators, etc. first. Think larger, deeper, steep sided, bottom drains, external pumps...
If you just want plants shallower sloped or stepped designs are fine. You can get by with smaller or no pumps.
If you want sound you can do pondless waterfeatures that are easy maintenance.
This is my pond when it first went in (the current iteration - this was a total yard re-vamp). It ~14' long x ~10' at its widest and ~5' deep at the deepest with plant shelves at the edges. It is straight sided so that herons/egrets and raccoons don't have easy access.
And here it is today - a little messy and green since it is very warm right now. The goldfish are hiding in the depths where the water is cooler. I started with 6 4" goldfish and now have too many to count ranging from ~10" to 3". Goldfish breed even faster than rabbits...