I'm guessing, based on my own corn snake experience, that people consider 40 gallons too much tank because they are rather laid back and prefer to bask or burrow unless they are being fed or handled, and it ends up seeming like a waste of space (plus its harder to heat properly, etc., which what is probably really causing problems). I've only had one as a pet, but have housed quite a few temporarily between owners and while moving wild ones from more urban environments to relative safety.
As long as you get a long tank vs. a high tank (corn snakes can climb, and will--especially to "hibernate", but they mainly hang out on the ground in the wild to chase their food), a single adult can be fine in a terrarium bigger than 20 gallons in size. Its been my experience that most habitats (as long as the reptile can move around comfortably into the different "zones" they need to access) that size is less important than things like a proper thermal gradient, a basking site, hiding box, etc. For a corn snake, if you have the right temp zones, a hide box in each temp zone, a water bowl large enough the snake can curl up into and submerge in without spilling, a basking site, I'd just shoot for an enclosure as long as the snake will likely get, and as wide as you can find and practically manage for that length. All the better, if it is tall enough to have a ground zone and enough space for a good sized branch and maybe a ledge or two.
I'll also say that I'm not a fan of live feeding captive snakes that will take pre-killed prey. One, pre-killed prey can be killed more humanely than being terrified and eaten alive, and two, I've seen captive bred snakes severely injured by live prey. I know this isn't something you brought up, but its something I've seen advocated when people want to provide as natural an experience as possible. There's really nothing natural about breeding and raising snakes in captivity--corns are easy because they are adaptable and tolerate a wide variety of conditions in the wild, which makes them less picky in captivity...and part of the reason they live so long in captivity vs. the wild is because we remove many of the hazards they would encounter otherwise (pathogens from prey, injuries from prey, predators, extreme conditions, etc.).
This is actually a really good info site for setting up an enclosure for a corn snake:
http://reptileknowledge.com/care/corn-cage.php
I also like this guy, who is an actual herpetologist:
http://blogs.thatpetplace.com/thatre.../#.W9hnw01Rd7h
...they differ a little bit in their opinions, but the areas where they disagree are pretty trivial with regard to the actual health of the snake. Unfortunately, there's a lot of bad reptile advice out there...which shouldn't be confused with opinion on what is better/best for the animal. After all, our interpretation of how they are doing outside of survival is pretty subjective to our interpretation--its not like we can take a survey of pet snakes everywhere!
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