Before you can calculate a resistor for that final circuit, you need to know the internal resistance of the driving source. What if the source had 0 Ohms internal resistance. Then it wouldn't matter WHAT value you picked, the source would produce infinite energy if that's what it took to fry the circuit.
So, once you have an internal resistance value, you can just use Ohm's Law to figure out how you want to clamp the output. How high can the source voltage get? When the transistor turns on, how much voltage is dropped by the internal resistance based on the value of the collector resistor.
As the circuit is drawn, the transistor is working as a crowbar overvoltage device. It flat out short circuits the source when the voltage goes too high.
After you calculate the resistor values (internal and collector), you can calculate the collector current. Then you can figure out how much power is being dissipated.
As drawn, nothing really happens except for a short circuit. A better circuit would have a very fast acting fuse in the lead from the source. Once the crowbar is tossed across the + and -, the fuse blows and removes power.
What you have is not so much an overvoltage limiter but more like a overvoltage crowbar.