Any excess capacity is dumped as heat by the inverter.
The size of the panels depends makes the capacity of the system.
40 two hundred watt panels would make 8Kw (40*200=8,000)
40 two hundred and fifty watt panels would make 10kw.
40 three hundred watt panels would make 12kw.
But THIRTY THREE300 watt panels would enable you to have 9.9 kw of output
Now you need to read other posts about 10kw systems as it appears that AGL has a limit on systems and the amount of money you can receive as feed-in.
You should also be aware that Fronius inverters are known to be very noisy and are in the top price range of inverters.
You also need to find out what is the maximum output that your Supplier (the company that delivers your electricity to your residence) may have in your area. For example in regional NSW there are limits of 3kw applied that I already know about.