You mean March flies or horse flies ?
1 - 3 cm's and give a dam painful biteOr are you talking about sand flies ?
At least in Sydney the mossies are only small (up to a couple of cm's), some of the tropical ones(north Queensland) are impressive 5 - 8cm's but they are not to painfull when biting. Its the Ross river fever afterwards that is nasty (but thats spread south as far as Sydney in mossies).
Alex