This is the minimum size to consider if fish and plants are part of the plan. The larger the pond the less it costs per square foot. I personally don't have a strong preference. One is not easier than the other. The life expectancy is the same.
Flexible liners allow for more creativity and can be adjusted to fit most any area.
You can build a little larger pond with flexible liner than a ridged liner for the same cost. You have the choice of where to put planting shelves and they're depths of water. You can also dig out water lily planting pockets in the deep areas of your pond, eliminating pots sitting in the bottom of your water garden. Next issue I will talk you through digging the hole and installing the liner.