I agree on the smaller the better. We have a small concrete through that is about 2ft x 3ft and only holds about 2" - 3" of water, it has a protected cover for the float. No need for sticks or rocks in it for drowning animals to escape. We put it in the shade and it has never run the 500 gallon tank dry even in a drought. The tank is filled by catchment from the barn.
"Man is still a hunter, still a simple searcher after meat..." Robert C. Ruark