Why not consider % of transactions with a fee?

This way users have direct influence in block size by paying a transaction fee (above median).
With paying a transaction fee users have a vote in the 'free rider effect'/spam and if current levels are acceptable.

1. small block size -> only transactions with fee will be processed fast (others get parked in mem. pool)
2. big block size -> a lot of transactions have a fee and a lot transactions without a fee (spam?) are accepted
3. median block size -> a balance between transactions with and without a fee

Also added a bigger scale (due to near future (holiday) transaction peaks).

I used Gavin's proposal as starting point: