I'mnot sure, but wouldn't it be the same way we agree on the reward for a generate? By refusing to accept blocks that are too big?
Sure, but the reward sizes are set in stone in the protocol and follow a predetermined path. The maximum block size will have to adjust to balance security and transaction demand.
I think the max block size is set so that the chain file will never be larger than [max block size]*[time since first block]/[10 minutes]
If a generator could just increase it they could force a huge block on everyone. To avoid that, we all reject oversize blocks. I believe it would be a breaking change, but there are ways to get it done anyway. If file storage becomes absurdly cheap and many blocks are full we will organize the change by issuing all new versions for a while to trigger the rule change at some future block.
This is my understanding, anyone please correct.