How big are Australian parakeets?

There is no such thing as an Australian parakeet.

The term "parakeet" is a generalised term to cover numerous species of smaller, long-tailed parrots. The Word tends to be used in countries outside of the birds' native lands. For example, the term "parakeet" is never used in Australia to refer to budgies, lorikeets, rosellas or cockatiels, yet these birds are commonly known as "parakeets" overseas.