Maximum I2C bus length is determined by the capacitive loading of the wiring and devices connected to it. Being a 'pull-up' only system it relies on current flowing through the pull-up resistors to give a fast enough rising edge on the signal. Lowering resistor values will increase the maximum speed but also decrease power efficiency and eventually overload the devices on the bus.
The only way to extend it's range is to receive the signal and re-create it on a new bus segment. There are devices for doing that.
Brian.