The accompanying diagram shows a pond with a maximum length of 5 metres and a maximum width of 4 metres. The final measurement you will need is the maximum depth of your pond, the diagram to the right shows a maximum pond depth of 1 metre.
Liner width = max width + (2 x max depth) + 1 (for overlap)
So for a 5 meter by 4 meter pond it would be:
Pond liner width = 4+2+1 = 7 metres
You'd need a pond liner that is 8 metres x 7 metres