There is no such size as sufficient size.
a ten foot tall barbed wire fence is not necessary if a four foot tall mesh fence keeps out all the threats.
there is only so much that a miner overtaking the chain can do, so an attack for economic reasons is not feasible today.
bitcoin mining's network is probably an order of magnitude larger than needed to protect the amount and size of transactions that are processed each day.
Nobody knows how tall of a fence we need. The taller it is the more people will feel comfortable and join in as merchants, customers, and savers.
Nevertheless, I think letting the new chains have a subforum is fine. They will be canaries in the mine. We'll be able to see how double spends affect people who are actually trying to use a currency and how much power and in what exact ways attackers come.
The equilibrium is one chain imo and there is nothing compelling enough to make it not be the oldest, longest and strongest chain.