Why do it as an OP_RETURN output? It could be a simple token in the coinbase input script, similar to how support for P2SH was signaled among miners. And why should there be an explicit token for voting for the status quo? Simply omitting any indication should be an implicit vote for the status quo. A miner would only need to insert an indicator into their block if they wished for a larger block.

I don't really care the exact location it's put in. I just thought there wasn't an explicit need to put it in the header (via a bit of nVersion), and the scriptSig is already used for many things (block height, merged mining hash, "\"P2SH\"", miner identifier). And voting to keep the block size the same by not voting is fine by me.

That said, proposals of this type have been discussed before, and the objection is always that miners would want larger blocks than the rest of the network could bear. Unless you want Bitcoin to become centralized in the hands of a few large mining pools, you shouldn't hand control over the block size limits to the miners.

Yeah, that was the conclusion we came to chatting on #bitcoin-wizards the other day. I now think that this could be useful to dynamically increase a lower limit, but that there should still be a hard upper limit like 20 MB. I think that just changing the upper limit might be simpler and better, though.

- Stephen