Linda, there is no such animal, nearly all are small flowered or in the case of Phaius tankervilleae not grown for the cut flower trade.

What's wrong with in-flower phals in pots as table decorations and cheating a bit by add a few "Singapore orchids" to the bouquets?

As Arthur said, the problem is going to be availability. There are a few large flowered natives eg Phaius tankervilleae, Phalaenopsis rosenstromii, Dendrobium capitisyork and its hybrids and several Caladenia but you'd be hard pressed getting a florist to find them. As you're in NSW, have you considered Dendrobium speciosum (if it's a spring wedding?) The flowers are only around 1 1/2" but the spikes are sufficiently crammed with flowers to give any other native flower a run for its money. You'd probably need to get in touch with some larger native growers or native orchid nurseries in your area to source them but you may be able to come to an agreement between the florist and the grower. However, it may be very difficult assuring good quality flowers at the right time. If you're looking for native orchid growers or nurseries ask at the nearest ANOS group. Go to the links page of their website: http://www.anos.org.au

Personally, as weddings are a string of problems waiting to eventuate, I'd be inclined to forego the native orchids or bend the rules and use readily available exotics to save yourself an additional stress. Sorry for the chary, married-male response.

Andrew