You will need one cross sectional span for every metre along the perimeter of your pond in both directions.
You will also need enough rope to weave along the entire perimeter of your pond.
Case study (3m x 2m pond):
Long Span: 3m x 2m = 6m
Short Span: 2m x 3m = 6m
Perimeter: 3m + 3m + 2m + 2m = 10m
For a 3m x 2m net you would order a 30m rope.