It looks like a very simple problem.
Anyone can find number of rooms required based on size of family & Budget constrains.
Flexibility required by room layout for temporary needs; such as sleep over friends, birthday parties, visiting parents and friends. Many a times the number of, size of rooms and home is determined by the living standards and cultural requirements.
Number of rooms required, varies based on size of rooms and life style. In some cultures cooking is big part of life, then family will need big kitchen.Party lover needs good kitchen and connected party area. First home buyers like option of putting property on share accommodation for extra money during temporary lost income.
Old people look for sunny room and small garden and space for grand children come over in vacation.
The empty house feels bigger as compare to house full of furniture. The house with small size furniture and well lighted will give bigger feeling then it is.
One should think about existing furniture’s use in new house. At least think about multiple layouts on floor plan of the same furniture. If property offers non standard size of room but lay out is appealing then be ready to accept it.
One has o keep in mind the supply and demand will govern what will be build by builders. The population projects in Australia is provided by ABS
If you are planning to move inter-state, off load as much furniture as possible. This will reduce cost of transport and damages. Keep in mind the size of rooms and numbers of rooms should be such that it will serve the family purpose for at least five years.