In most countries, you cannot choose the length of your PIN; often it is limited to 4 digits, and e.g. some ATMs don't even wait for you to press enter after you enter the fourth digit (UK).
In Australia you can choose a PIN up to 6 digits, though the banks may warn you that a >4 digit PIN can be problematic in countries like, well, the UK I suppose.
edit: huh, it's actually more than 6. According to Commonwealth Bank, it can be up to 12.
