hey lizardsNturtles,
turtles are a longtime commitment buddy, and as the turtle will grow it will need more room or it can cause the turtle to get shell deformities, so the old myth small tank and the turtle wont grow much is false.
out door ponds are great idea natural but a 6x2x2 or 5x2x2 is ok for a breeding pair but the room volume of water the better. as for species long necks do grow slower as imalizard said so you can get away with the 3fter for a while and if you after shortnecks i reckon kreft or a murray would be ok for a few yrs.
good luck bud
turtle_alex