I use 8 inch PVC and 6 inch PVC in all my ponds with gate valves on the outside of the pond dams. Reason for the size is cost of product and access to the supply. I have drained my ponds and I am usually not in a big hurry. One of the engineering fellows will have to calculate the head and volume for you to size the pipe for a complete drain in 48 hours. I have seen some tables in some of the engineering books that I have had at one time but dont remember where. I usually purchase my pipe and valves from the same supply company that supplies our local water board their products. I always use PVC. I have observed a lot of failures in older ponds here that belonged to neighbors and family members that used metal pipes in their ponds.