My pond 7'x12' and between 1 1/2'-2 1/4' deep. And it is way way tooo small. I am in a area that might freeze the top two inches on a hard freeze. I am hoping, praying and wishing I will be able to get the 8'x16xmin 3' deep pond in place. Around two winters ago I was so happy we had pond. The power was out for almost a week and we have a well. We were able to use the pond water as gray water and use the toilets.
Back to size. Even if you built the pond the size of a swiming pool, after time it will seem small and want and even bigger pond.
With fish in a shallow pond, put some netting over it. You might not see the large birds around but they will see those fish. I know by experince.