Why don't we just let miners to decide the optimal block size?
If a miner is generating an 1-GB block and it is just too big for other miners, other miners may simply drop it. It will just stop anyone from generating 1-GB blocks because that will become orphans anyway. An equilibrium will be reached and the block space is still scarce.
I think this is exactly the right thing to do.
There is still the question of what the default behavior should be. Here is a proposal:
Ignore blocks that take your node longer than N seconds to verify.
I'd propose that N be: 60 seconds if you are catching up with the blockchain. 5 seconds if you are all caught-up. But allow miners/merchants/users to easily change those defaults.
Rationale: we should use time-to-verify as the metric, because everything revolves around the 10-minutes-per-block constant.
Time-to-verify has the nice property of scaling as hardware gets more powerful. Miners will want to create blocks that take a reasonable amount of time to propagate through the network and verify, and will have to weigh "add more transactions to blocks" versus "if I add too many, my block will be ignored by more than half the network."
Time-to-verify also has the nice property of incentivizing miners to broadcast transactions instead of 'hoarding' them, because transactions that are broadcast before they are in a block make the block faster to verify (because of the signature cache). That is good for lots of reasons (early detection of potential double-spends and spreading out the verification work over time so there isn't a blizzard of CPU work that needs to be done every time a block is found, for example).
This would make it very easy for a miner to fork the blockchain.
He would just have to create a Block that's so large that it gets rejected by half of the Network.
He could then fork one branch of the fork again.
This branch could be forked again and so on... until the mining power on one branch is so low that he could perform a 51% Attack on that branch.