by Mr. Whiskers » Wed May 24, 2017 7:30 pm
I'm not very knowledgeable with Mexican king snakes, but after doing some quick research and applying what I know about terrestrial snakes, I can say the following:
The general rule for housing a terrestrial snake (one that lives exclusively on the ground) is that half the perimeter (the sum of the 4 sides) of the floor should at the very least be HALF the length of the snake. If the snake is longer than this, then that cage is too small. That being said, if the enclosure is larger than this, and your snake doesn't show signs of stress from this (i.e. not feeding or being a lot more nervous) then there should be no problem.
At adulthood, king snakes typically average between 900mm to 1200mm (I'm not sure if your snake will be smaller or larger due to specific locality).
My personal opinion?
I don't think it would be a very good idea to transfer your snake from its hatchling enclosure straight to it's adult enclosure. If your snake is a species that is small at adulthood, then perhaps you can do this, but not for medium to large snakes. The size differences are typically quite large and can more often than not have negative influences on your pet. I would feel more comfortable to have at least 1 intermediate enclosure (a medium sized cage before the adult one).
600x350 sounds perfectly fine for a small adult.
I don't claim to have a lot knowledge on the subject, but I wanted to help (this website is rather quiet and empty). If someone with more experience to give advice reads this, and has something to add / correct / an opinion to share, It would be appreciated.