Before digging or building a pond you have to figure out how much run off you have to fill it, and keep it full. There are sites that you can search to figure this, but the basics are how many acres of land will receive rain that will flow into your pond? How much rain do you get? How much of that water will flow along the surface? there is a HUGE difference in the amount of run off you get from pasture compared to woods. Once you know how much water you get from the rain, you can determine how big a pond you should have. While there is nothing stopping you from going bigger, the is only so much water that will go into the pond.
I have two ponds. One is 3/4 of an acre and it receives water run off from about 20 acres of land. On a heavy rain, I get well over six inches of water into the pond for every inch that it rains. My other pond if 4 acres. It receives water from about 40 acres of land, but a big chunk of it is heavily wooded, which reduces my results in the pond. I get about 2 inches of water in the pond for every inch of rain I receive. If it's been raining for awhile and the ground is really saturate, that has been known to double. If the ground is dry, then I don't get any run off on smaller rains and you can't even see the increase in the pond level.
Just about every time it rains, I go for a walk once it stops to see where the water if flowing. I build my roads to catch and funnel the water to my big pond, and I add ditches to increase the amount I can get there. I have one more area on my land that I'm losing water that will require three culverts and two long shallow dams to get the water to go to where it will flow into my pond. This will add everything that my small pond is getting when it overflows, plus another ten acres of land that I'm losing out on right now. Once I do this, I'm done and will have to live with what Mother Nature gives me. Fortunately, on a normal year of rainfall, about 40 inches, that is plenty. In fact, I'm fine as I am, but what I'm worried about is those dryer years and extra hot summers.
Eddie