The best way to choose what size to use is to see what is most commonly used outback, skinny tyres on split rims where ( until recently) most common.
Nowadays eqiupment for fixing tubeless tyres and plugs that can fix a puncture without even taking the tyre off make things much easier that to strip a tyre off a rim and to patch a tube,(and hope you got all the sand out of the tyre)
I run 235/85 16's split's on my cruiser, and its most definatly not for looks.
They are a 10 ply tyre, have a load rating of 120 and where less than $250 fitted with tubes.
I'm about to up date to tubeless tyres, new steel tubeless rims will cost abit over $100 each, but I will retain my 235's .
I'm about to put on some more cheep mudddies on ($200), I bought 2 that have done 10000ks that seem to be wearing OK, but and this is a big but, as far as wet road holding goes, they are SHOCKING, but in the mud, the're great, will put another 2 on for Easter fun, as far as outback travell goes, I don't think so, I don't trust them for that kind of work, any future outback travells will be on the bridgies which is what I have faith in
Shane