If you're building your own pond, the pond liner is a very important component. To figure out how large of a flexible pond liner you need, calculate the width, depth, and length of your pond. You will want to be sure you plan for a minumum of a two-foot overlap around entire pond. Basically, you need to be able to cover your entire pond base, and have enough left over to create an overlap around the perimeter, which would be covered with rocks to hold down the pond liner. Many people install underlay to provide an insulating layer between the soil and the pond liner itself.