BIP 100: What votes are possible?
Gregory Maxwell in IRC:


  the scheme would have the pariticiants specify the size they want, and
  at the end the period the smallest 25% would be tossed and the
  smallest remaining would be used.


So in each block there would be a field, initially set to 2 MByte. A miner is free to set any value (up to 2x the current limit).
Assuming we have the following values set in the last 12,000 blocks:


20% 2 MByte
20% 3 MByte
60% 4 MByte


Then, because only 60% want 4 MByte but 80% agree to have 3 MByte, the limit is increased to 3 MByte.

There would be a new vote every 12,000 blocks, and a decrease also would be possible if that is what the miners want.