obviously the bigger the better but as i said its been done before. Yeh i have 3 northern snappers in a 3 footer no problems at all they love the room to swim around.
You might want to have a look at some of the good information that is available on the internet. This is a good starting point: http://vhs.com.au/pages/Turtleintro.html
go a 4 or 5ft if you really want your turtle to be happy when he growns up, the turtles i have will be going into a 6 or 8ft but thats coz they grow to 50cm shell length and females can get to 85cm head to tail, but yea, get as big as you can make room foor, your turtles will love you for it
My pair of Macleays have a 4x2x2 tank, I wouldnt go any smaller for adults. Dont forget u need a basking area and all for them which starts to eat up some space.
i have two murray river turtles in a four foot tank, my tutles are about 15cm
IMO i deffently think bigger tank the better (according to their size) as soon as my turtles reach 2-3 years im wishing to get them a nice pond or something.
BTW you really need to do some research, these are special creatures with special needs
Turtles aren't like snakes in that you need to start them in smaller tanks. My hatchies went straight into a 3 foot tank and are doing fine (although I sold all but 1). The only thing is that they dont tend to come out of the water as youngsters. And you also have years before they are too big.
But with Macquarii in NSW the only subspecies recognised until 2004 was Macquarii Signata (Brisbane River). The other Macquarii were all the same. Now they are different and they also have the Sydney basin one that I believe John Cann has decided were merely escaped pets. (Macquarii dharuk). But you would need to be careful to make sure you got the right species. Otherwise you will end up with a giant.
The problem we have is that there are only 9 on the NPWS licences as at the 2005 returns. That makes them rarer than GTPs (there were 111 of them). However there were 1210 Emydura Macquarii many of which would be from northern NSW. I have a mate who has them (I Know because I actually got them from the Clarence River in 1990) and I guess would sell any for about $100 each. But that's just a guess.
I can give you a male long neck with a 10cm shell.