i would say start it in a 20 and it will be fine for possible 6months depends how fast it grows...its always cheaper to use kiddie pools and rubber maids you get bigger enclosure for a way cheaper price that way...just food for thought
Have you considered a musk turtle instead? They don't get very big as adults and might do quite nicely by itself in a 10gallon for its whole life, depending on how you set it up!
I just got 3 little maps. They are in a 10 gallon for now. At 3 months I will move them to a 20 long. Then it is all dependant on how fast they grow. They don't grow like sliders do. They will eventually go into a 140 gallon pond.