How do I work out the size of pond liner for the hole that I have dug?

Double the maximum depth and add it to the width and length.

Depending on how the edges of the pond are to be finished, allow some spare liner to either sit beneath coping stones or to form planting pockets. Often rounding the liner size up to the pre-determined roll width will give you ample to spare - for example, a projected liner size 4.4 x 5.6m can be rounded up to 5 x 6m giving an overlap of 0.6 and 0.4 metres respectively