I absolutely believe it is !

If fed appropriately and healthy, a snake will reach its genetically predisposed size. Which can vary for each snake somewhat.

There are variables, though. Over and under feeding can also affect size. If you overfeed a snake, it will not get "bigger" then it should, it will however get "fatter" overall.
With underfeeding a snake can end up "stunted" and skinny.

In the wild, different regions can affect size as well. I used to have some WC Ball Python rescues that were HUGE. Not just huge as in weight, but their heads were much bigger, too, and they were larger yet very fit for their size. (not fat) It has since been established that BP's from the Volta region seem to be larger, genetically.

I bought 5 hatchlings in 2010. None were related. All were fed on the same diet and very conservatively. No power or fast feeding, since I wasn't interested in breeding them. There are 3 females and 2 males. One of the males grew far faster and bigger then any of the others. To the point where I thought he must be sexed wrongly. He grew bigger and faster then the females were. So I sexed him this year and yup, he is definitely male. Just a really big one.

So yes, size is genetically predisposed.