Tank advice

[Kierongrady]

Hi there. I have two red eared sliders that are about 2 and half years old. Their shells are about 6inches in length. I think they are going to continue to grow although am not sure by how much. They are presently in a 100 gallon tank which seems fine for them but I expect they will eventually outgrow it. I don't want to keep buying bigger and bigger tanks. I would like to just get the "final tank" that I will need now. My question is "what size tank should I get if I want it to be the last one I do get"?
Thanks for any advice people have.