Actually....it more a matter of them finally having room to grow to their natural size.
Carp/goldies kept in small bowls and tanks will stay small, not because they grow to their surroundings, but because they end up stunted. Limited space, poor water quality (a goldie will produce massive amounts of ammonia in a bowl within hours of receiving fresh water) and limited oxygen availability all end up weakening the fish so it doesn't grow to it's natural potential or even live out a semi-normal life span. Most goldies are very capable of living 20 years or more if their basic requirements are met!
In a big pond like this, you can expect common (feeder) type goldies to grow to 12-15 inches long. And, if you got koi...they could double that.
If you did get koi...PLEASE restrain the desire to get lots of them. Even a pond this size won't hold a bunch of them...but 2 or 3 might work out ok.
If not koi...then rest assured you will still have lots of fun watching all those baby goldies grow into mammoth fishies.
You might want to check in at your local, privately owned fish store and see if the owners need a supplier for goldies (or if they will at least take your extras off your hands.) You will end up needing that backhoe again in the
spring otherwise.
You may also have an area fish club that would take extras too.