1. A general guideline is to find the adult size of your turtle and multiply the turtle's adult length by 5. That's how long the aquarium will need to be and will give your little friend plenty of room for doing laps.
2. Take their adult length and multiply by 2.5 and that'll be how deep your water should be. This gives them plenty of room to move about freely and get some exercise, as well as allow you to place a basking area half way up and prevent your turtle from trying to escape. They are great climbers!
You should house your water turtles in a minimum of a 20 gallon long aquarium. This is a minimum recommendation, bigger is better. Your aquarium should have at least enough water to be as deep as the width of your turtle. You can add more water depending how good of a swimmer your turtle is