You need 10 gallons of water per inch. RES get "the size of a dinner plate" according to www.anapsid.org
. You're looking at a 200 gallon tank, or an indoor pond. Sorry. 20 gallons will work for a couple months, max.
I have One Eastern Painted Turtle about 5" long, in a 75 Gallon Tank with a bunch of Guppies (that he eats). He is 11 months old (my daughters caught him between his egg and the water). Started at 1" in a 10 gallon, then moved to a 20, then to the 75 because I could not keep the smaller tanks clean (and I finally ran across the 10 gal/inch rule). It takes every bit of flow from an eheim 2217 to keep it clean.
He'll be going back to his point of collection in June.