The rat snakes are a diverse and widespread group who, as the name suggests, feed extensively on rodents in the wild.
Whilst most commonly-kept specie - such as the corn snake (or to give it it's alternative name the "red rat snake"), yellow rat snakes and black rat snakes are of US origin, other members of the group may be found as far afield as Canada, China, Japan and Italy.
With such a wide variety of species, so it follows that a range of different adult sizes may be experienced but fortunately, at least for the North American examples, care is reasonably uniform.
The corn snake is discussed in depth elsewhere on this site so I do not intend to repeat myself here though the basic guidelines for corn snake care apply hjust as accurately to the other common rat snakes.
As you might imagine the black rat snake tends to be a uniform black in color whilst the yellow rat snake is a dull yellow in color.
In my experience the yellow rat snake may reach a size quite a bit larger than it's cousins - up to 2 metres is not unusual - and can prove quite aggressive if you choose the wrong specimen.
It is highly recommended that you attempt handling any rat snake before purchase though also realise that some snakes may change their personality as they grow.
I once had a perfectly lovely hatchling corn snake that turned into a complete beast after a few months making him virtually unable to be handled.
These things happen but you just have to take it in your stride and weat chain mail gauntlets when cleaning your snake out (kidding!).
Generally a temperature of 26 - 28'C will do fine for rat snakes, though as always a temperature gradient should be present to allow your rat snake to control it's own temperature.
Rat snakes will generally reach sexual maturity at 18 - 24 months of age if well fed and will breed in captivity without too many problems.