The capacitor size corresponds to the time that it must supply the sensor (i.e. during the time any slave is transmitting) and the amount of current the sensor draws. The supply voltage will obviously droop during this time; if the sensor is not dependant on supply voltage it's not an issue. If it is dependant, you'll need to provide some regulation/stable reference.
So far as working out the size, CV = IT (capacitance * voltage = current * time, in SI units). e.g. 0.1F (a common supercapacitor value) charged to 5V will drop to 4V in 33 seconds with a 3mA load, so 0.1F is more than enough assuming the master recharges the cap at least every 33 seconds and that the sensor works with a voltage in that range. 1000uF will drop a volt in 330ms. It depends how long a slave takes to transmit its data before the master starts driving the bus again.