As per advice above, you should not have to worry too much about size in most places, however make sure you find out what size the camp sites are.
The first time I took my fiancee & her 3 kids camping we had a 6 person dome from Supa Cheap:
As you can see we would not have had room for a 12 person/3 room tent on that site
we had the smallest spot & when I rang to book the NPWS ranger said he could not give many any size's because they had only just renovated the camp ground & had not got a site map made yet, he asked how big my tent was & he assured me there would be no probs.... lucky we only had a small dome
Since then we have bought a 12person/3 room tent & it is the only way to camp, especially as mentioned when you have 3 kids stuck indoors with torrential rain outside, as what happened to us LINK: http://forum.camping.com.au/viewtopic.php?f=62&t=2975
The van park at Gulgong only really had one big campsite & lucky for us we got it, for two reasons, size & good drainage