3 feet, then I would recommend getting a corn. As it is the smallest of the rats and usually averages around 3.5' - 4'. The rest of the rats all will reach 5' easily and some even hitting the 6' and 7' mark!
Here are some of the most common rat snakes in the hobby and the average size as an adult:
Black rat snake - 4.5 -5.5, 7' specimens are not uncommon and the record is 101" Color is variable with many morphs available.
This is my KY locale black rat..

And this is my MO locale black rat...

Yellow rat snake - 4' - 5', record length just over 90". They are also available in many morphs.
My normal yellow rat snake...

Everglades rat snakes - Same size as the yellow and also in a few morphs.
My normal everglades...

Gray rat snake - average between 4' - 5' and can attain lengths of 72" and the record is 84". The white oaks phase is the most sought after coloration of the gray.
My white oaks phase gray...

Texas rat - average between 42" - 72" and the record is just over 7'. The texas rat has the stigma of being the most aggressive and the worst biter of the rats. Although I think this is just a false presumption. All rat snakes have the potential to bite, given its mood and personality.
The texas rat comes in lots of morphs as well.
Corn snake - Average adults range from 18" - 44", but the record is 72"! The corn comes in the most morphs available and is the most frequent bought snake as a beginner snake. There are a multitude of color variations and patterns available for the corn, and this is why it is so popular. It is relatively cheap as well.
My normal KY locale corn...

Hope this helps and good luck on what ever you decide on getting.
Brian
>>Hey guys, i was considering buying a Leucistic rat snake and was wondering how big do they grow to? I dont want a snake much bigger than 3 feet. Also, how are they behaviourally (friendly, mean, good eaters, not good eaters, etc.) Finally how big of a Vivarium would i have to build (dimensions) to house one? Thanks so much for your help!
>>-----
>>-Doug Daly
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RATS
1.0 Corn snake "Warpath" (KY locale)
1.0 Black rat snake "Havok" (KY locale)
1.1 Black rat snakes "Reaper and Mystique" (MO locale)
1.0 Albino Black rat snake "Malakai" (Dwight Good stock)
1.0 Everglades rat snake "Deadpool" (Dwight Good stock)
0.1 Greenish rat snake "Rogue" (Dwight Good stock)
1.0 Yellow rat snake "Wolverine" (Dwight Good stock)
1.0 Grey rat snake "Punisher" (White oak phase)(Dwight Good stock)
RACERS
1.0 Eastern Yellow Belly racer "Nightcrawler" (MO locale)
KINGS
1.1 California king snake "Bandit and Moonstar" (Coastal phase)
1.0 Prairie king snake "Bishop" (KY locale)
0.1 Black king snake "Domino" (KY locale)
1.0 Desert Kingsnake "Gambit"
0.1 Florida Kingsnake
MILKS
0.0.1 Eastern Milk snake "Cable" (KY locale)
0.0.1 Eastern/Red milk intergrade "Omega Red" (KY locale)
Good luck and Happy Herping
Brian