Ok, so I don't claim to be an expert on anything. I do claim however to have grown up around snakes and do have some knowledge about them and their growth habits. First off, a snakes environment has everything to do with how big it gets. Does this mean that if you keep a snake in a small cage it will stay small? No. It will get sick and most likely die. What if you put it in a huge open cage you say? Well, then the snake will become agitated (depressed almost) and quit eating. Once the snake quits eating because it has no place to hide it will die if you don't release it, or put it into a tank with proper hiding accommodations. So what is needed for growth of the snake? That's simple, and the main reason I say that environment has everything to do with how big a snake gets. The size of the cage/ environment (I'm including wild along with domestic) determines how much the snake will move and how much it'll want to eat. The amount of heat determines how fast to food will digest. Warmer temps means faster digestion, which means more food, which means longer body. However if it is too hot the snake will receive skin damage and again may stop eating all together. The type of foods available can determine how fast it will grow as well, seeing as different animals provide different types of nutrition. Yes snakes can have bad eating habits just like people. And the predators in the area will determine the life expectancy of the snake thus determining the size it grows to. How big can they get. Well in all honesty, we really don't know. Lots of people think they know, but they just know the norms, of mostly domestic snakes. Please consider the vast amount of stories of giant serpents dating back thousands of years. I strongly support that if there are that many stories some from other sides of the world thousands of years back when there was no communication amongst the different tribes at that time. How could they tell stories so similar about giant snakes if they didn't communicate with each other. Also, I've been to many different places where there evidences and stories of snakes that far exceed that of what the records hold. For example I have a friend from Papua New Guinea who sent me this picture. http://www.picable.com/Abstract/Giant-Snake.3037803
(Compare the snake to the 6' man beside it.)
This particular snake is indeed very large, but there are stories of snakes even larger than this that have been killed. There are snake skulls there that measure almost 2feet in diameter. Anyways, I just wanted to clarify, that though we haven't caught a living ancient snake that measures 60'+ we can't say that this type of snake doesn't exist. I'd like to offer one last evidence to this. Consider the tombs we have found of 9 to 12 foot people in recent years. There are sometimes people who far exceed the norm. I present to you that this principle is true no matter what the species might be.