A new BIP being discussed on the Bitcoin-dev mailing list is aiming at making variable Bitcoin block size depending on the fee the sender is willing to pay, the BIP is aiming at making the block space small, decrease the impact of spam transactions, balancing fees with smaller fees increasing in a higher percentage than higher fees, allow larger block size if the sender is willing to pay for it, allow wallets to display the amount, and price, of free block space left and allow senders to have more control on their fee/priority structure.

The implementation of this scheme is fairly simple, it starts by sorting transactions by fee in satoshi/byte, then ascendingly summing up the bytes of space used so far and checking if it is smaller or equal to the formula result, if it is not then the block size is not valid. This implementation requires a soft fork if the old hard coded block size limit is kept and a hard fork if an 8mb block limit is enforced.

The idea is still currently being discussed on the Bitcoin-dev mailing list as some developers have doubts about validation time, breaking child pays for parent and CoinJoin, you can follow the discussion more closely here.

References:

• Discussion
• BIP Proposal
• Child Pays For Parent
• CoinJoin