I'm not extremely familiar with Aussie Stock horses and their proportions, so I can't give you a really great answer. But- if you go to a good tack store (some have no idea what they're doing) the employees should be able to judge the basic size for you. For English bridles at least, it should be reasonably snug, but not tight, plenty of room with the throat latch. You know.
I would suggest full size. Fortunately bridles are pretty adjustable. Once you measure your bridle and have an approximate size, ask the company you are buying from what size would fit your horse by giving them the measured number.
Hopefully that helped.