I'm not enitrely sure what the smallest species are, but Painted turtles don't grow very big, about half the size of a full size murray river turtle. This was my setup:
Basically, you need substrate (calgrit is recommended), something that allows the turtle to get out of the water and bask under a uv light, a good filter (turtles can be messy), a water heater, and some plants and things for your turtle to tear to shreds and have fun with. I also had various fish, which he pushed around with his nose, but never ate, although other turtles will eat them. The water should sit at around 24 degrees, but up to 26 is ok, though no hotter. The basking spot should be warm enough that your turtle is happy to leave the warm water and bask, but you may never see it do this! It doesn't mean he/she doesn't bask, they just do it when you're not around. I'd often come home to find little wet turtle footprints across the basking rock.
Turtles also require UV-B. Most UV lights have UV-A, so you need to make sure you get one that also has UV-B, slightly more expensive, but worth it. Also remember to replace the bulb every 6 months.
I had various setups as he grew. He began in a 2 foot tank, with a few cm of water, and progressed rapidly to a 4ft tank, half full of water.
No idea if you can get painted turtles in nsw as pets, but they're pretty good little guys. They might grow to around 20cm? a pair would probably be ok in a 4ft tank, so long as they had plenty of swimming room.
Anyway, I have to get back to studying. If you have any questions, feel free to pm me. I'm quite happy to share all the information you need to know about caring for them. Haha I know a lot more about turtles than I do about snakes!
And good luck with it. They're fantastic pets.
