Trying to buy a pond based on average size would be like purchasing a home based on the size. You can find an average size if you look it up but it can differ from area to area.
Like houses, the sizes of ponds vary based on the factors listed previously: streams, shape, preferences, etc. It’s going to be different for everyone, but that is part of what makes ponds and water features so awesome – none are exactly alike.
Even if a pond is an average size, it’s not average because each one is unique. The part of ponds that don’t change often is the average depth.