Start here: Quality Farm Dugouts
When doing this for a farm, the usual calculation (Central Alberta Climate) is to plan for 2 years use, plus 2 years evaporation.
Dugouts should be as deep as practical to excavate to minimize surface area. That usually translates into 15 to 18 feet which is what most trackhoes can handle. Here a trackhoe costs you about $500 to the point where they unload. In a day's activity they can move a bucket load -- 2/3 to 2 cubic yards -- every 15 seconds.
Dugouts are almost never filled by roof runoff, but rather by land runoff. The usual figure used is that there will be at LEAST 1" of spring runoff. This is a low estimate. 2.5" is more typical. But that means that a 15 foot deep 1 acre pond needs 15 * 12 = 180 acres drainage to fill it. This ratio will fill it 95% of the time. There are charts for this ratio for various parts of the province.
Here in Alberta we get about 20 inches of precip per year, but the net level drop from an open body of water is between 36 and 48 inches per year. (This is mostly evaporation, but also from trees on the edge.)
Here, if you are going to make a dugout larger than 6000 cubic meters (200,000 cubic feet, 1.6 million gallons) or if wish to site it or fill it from an ephemeral stream you need a permit.
One thing to check is how much water you are using right now. Off hand 15,000 gallons per year sounds too small. The usual standard is something like 100 g/day/person, but that includes a lot of flushing. But at that rate one person is using 36,000 g/year and that doesn't count exterior use.