While the record Corn Snake is 6 feet, it was a wild caught snake. I know of no morph of corn that has even gotten that long (5 foot corns are uncommon, and 5-1/2 footers even less so).
As already stated, genetics plays a role. Keys Corns and Miami Corns are smaller than average. Some people consider Okeetees the largest type, but I think they've been selectively bred for so long that they may not grow to the sizes they once did a few decades ago. It seems morphs and selective breeding have resulted somewhat smaller corn snakes (on average) than wild caught types.