
Here is my pond. Not the thing to use as a centrepiece for proud gardeners. I think I need to thin the weeds out a bit now though, but I was letting nature take it's course. It is about 5' long by 3'-6" wide and 14" deep at the centre.
I just dug a saucer shaped depression in the ground, covered it with black polythene and mixed up stiff mud to cover the polythene with a 2" layer of soil to make it more naturalistic. I made it about a year ago and just filled it from the rainwater but. The only thing added was that small bit of duckweed, frogs, water boatmen, pond skaters and now Damselflies have all arrived of their own accord as it has matured.

I kept the width down to 3'-6" for close-up photography so I could reach the middle from either side, the length does not matter.
DaveW