There are flow rate charts on the internet on various pipe sizes and water flow, but you have to know the head pressure like AP said.
How large of a pond will it be? That will also have some bearing on how much water you need to move thru the pond. Cecil moves 50 gpm thru his pond and it holds trout during the summer. Deeper is better, more cold water reserves.
I have sandy soil. I looked into a elastomer liner and I found that clay was cheaper, but still too expensive for my pocketbook. That might not be the case with a smaller pond.
