There are two approaches to system sizing:
Approach 1: The solar-lover. Just bang as many panels on your roof as you can fit. You know solar power is a good thing, and if you are going to get solar installed you might as well cover your roof! That was my approach, and my last bill was $71.
Approach 2: Common Sense. You want to maximise the financial return from solar energy, and get the system size that will give you the optimum return. How to do this is a long answer. It depends on how much electricity you use and even more crucially when you use that electricity. I explain the concepts you need to know about careful system sizing here.
If you want a solar installer to do the maths for you, then you can get 3 independent quotes (with your paybacks calculated) here.