In assessing the future size of koi, you don't have a chance with a retailer. Go to the breeder.
First - look at the parent koi. The likelihood of growing large is much more with bigger parents. And older parents.
Second - look at the siblings of the koi you like. There are always a few "tobis" which grow much faster than their siblings. Usually these fish are not as pretty - because they expend their energy in growth instead of color. It is thought that the tobis get bigger because they have learned the delightful flavor of sibling sushi. Tobis will always be bigger than their siblings, tho - so I think that's not necessarily the only truth about why they are so much bigger.
Third - look around.
There is so much to learn at a koi farm. Most koi farms are pleasant places to be. Enjoy your trip.
Keeping a big koi for a long time is a task that requires a variety of skills and dedication. They aren't goldfish that you replace every three or four years even if they have been well kept. A good, well kept koi, can live 35 to 50 years, and there are reputable reports of some living much MUCH longer.
My longest koi is approaching 40 inches. My biggest koi is about 40 pounds.
They came from reputable sources, most are from the Dainichi bloodlines, after long discussions with the breeder.