To work out how much water volume (note that's water volume which is different to tank volume) your turts require you need to measure them, you do this the same way as you measure tortoises which is to measure the Straight Carapace Length or SCL. The correct way to measure is in a straight line from the front edge to the back edge of the carapace (upper shell), ignoring the curve of the shell and the head and tail.
When you know the SCL (in inches) multiply it by 40 to give the minimum water volume required, eg a 2" turt will require a minimum 80 litres, a 3" 120 litres, 4" 160 litres etc...
If you have more than one turt then allow 40 litres per inch for the biggest one, plus 20 litres per inch for each additional turt.
Remember that these are minimum amounts, more is always better!
When you come to select a tank you have to bear in mind that in most cases you won't be able to fill it right to the top, this is because they need dry land to bask. If you are a bit of a DIY person, you can build a basking spot above the tank to allow more water volume. see here (scroll down to basking area) -
http
/www.austinsturtlepage.com/Care/housing.htm
i would reccomend you have a good look through this website, it is an excellent source for details:
http
/www.austinsturtlepage.com/