You want the pond outside to be 100 gallons or more. The sun will heat that water up fast if its not big enough. So you usually your way past that that rule anyway but you dont want to many turtles in one pond.
i personally like liner ponds better. u can make them huge and make them look ho u want. although the preformed ones may be a little easier to deal with
65 Gallon -1 RES (Myrtle) -1 Pleco (unnamed) -1 four lined pimelodus catfish (unnamed) -1 shubunkin (unnamed) 29 Gallon Planted Tank -3 cory cats -# guppies!