The rule of thumb is 10 gallons for each inch of turtle (measured from front of shell to back, not including head or tail)
example: a RES that is 2 inches long would require a minimum* of a 20 gallon tank
Keep in mind, that your cute lil baby turtle will get bigger! A lot of people believe that a turtle will not outgrow it's enclosure. That is only caused by a less than adequate environment, paired with malnutrition!
A male RES can get up to 9 inches long, while a female can grow to 12 inches. That means ONE adult female RES will require a minimum* of 120 gallon tank!
Your RES is a big investment of time and money so be prepared!
Good luck to all!