There's a calculation as to how much tank you need per RES, which is measured based upon the size of the turtle.
Your RES, god willing, is going to live for a very long time and will grow to about 2-3 times its current size as you describe it.
I've had to move my 14y/o [roughly mid-life] into a tank measuring roughly 2 feed by 4 feet with a depth of 3 feet. We would like to give her a larger habitat, but that would require a custom fabrication or more permanent installation.

I'd recommend getting a MUCH bigger tank now, rather than buy several in the future.
Nobody realizes the commitment when they buy that silver dollar sized cutie pie...