RES grow very big. If she is placed in a proper size enclosure, and fed and heated properly, she has a chance of catching up to the size she should be. A larger tank would be much better and a pond would be best.
A lot of people buy a prefabbed pond and put it in their basement. Or they build one themselves.
You could also get a big stock tank the kind used for water for live stock.
I would do that, but we don't have a basement, also after hours of testing what she likes, she likes to have people around and likes to be on the land, and she tolerates handling well. Plus, through hours of training, she doesn't bite anymore she just pees on you if she does not like you.
Then shoot for the biggest aquarium possible, at least a 100 gallon or to do it cheaper, use a live stock stock tank as the aquarium and you can build a "deck" on it for land area. It may not be needed now, but will in the future. Plus needs food filtration as the RES need to eat in the water. The RES is one of the largest species of freshwater turtles.
Oh, okay, I want to buy a 400 gallon tank and do everything I want to do(bio-active and multiple RES with automatic feeding and lighting with all the extras), but I do not have the space or money to build this habitat.
That's why I said go with the stock tank instead of the aquarium. Stock tank a lot less for the same amount of gallons. As for the deck, I'm not talking like a human deck. But a tortoise deck over one end where she can get out to bask.
If you check out CL or FB Marketplace you might be able to find a used large enclosure even cheaper.