You wouldn't speak about resistance of the transistors and SCRs. The transistor is a current source and the gate of the SCR (as far as remember) is to be seen as a diode (or zener diode).
The resistor is only there to protect the transistor/SCR at abnormal transient occasions, as I already mentioned.
If the transistor is on by the optocoupler and the SCR is off because no voltage is present, a problem could occur if you suddenly connect the AC power at the peak of the sine wave. Then it will be a race between the avalanche function of the SCR and the rising edge of the current through the transistor/SCR (di/dt). That is a very fast process. The quicker the SCR switches on, the less current the transistor/resistor have to take.
Often Snubbers and serial Inductors are used with SCR's and Triacs. They help removing disturbance and prevent self firing, but also reduce di/dt (du/dt).
In the normal case (AC voltage present when the optocoupler switches), the resistor would not be needed! That's because the transistor will switch on rather slowly. Thus, when the transistor/resistor/gate current reaches max 40mA, the SCR will "take over" all current and practically protect the transistor circuit.
I suggested a 0.6W (metal film) resistor. It should be well able to handle the high current for the very short (micro seconds), transient moment. Just don't use "fusing resistors"!
In the normal case, you have the max transient, power in a 100 Ohm resistor of P=40mA*40mA*100=160mW.
Of course the RMS power is even much less!