If I actually. Do want to build a very large pond outside let me know I have built a few for paying customers and also have some neat ideas on the filtration end for u to use. Very diy. Gotta say largest. Pond I've hand dug was only 14 x11 and 3also feet deep but. The ground was extremely rocky. If it was more of a sand you could easily dig. A very large pond with a shovel maybe to the order of 40 square feet 5deep feet deep everyday .
Take a look at cal torpedo pumps lords flow for the price of it.
I have a ~35,000 gallon pool in my back yard. It is about ~35 feet long, 15 feet wide and goes from 4ft to 10ft deep.
It took the pool company about 3 days using a backhoe and other assorted earth moving equipment to dig it. Using a shovel will take you several weeks of 8 hour days shovelling to do the same. 30,000 gallons is about 4000 cubic feet or 148 cubic yards. This picture is 6 cubic yards so you'd need to move about 25 piles that size.