can't I accidently destroy it using wrong resistor?
You can destroy the LED if the value is too low. But according to the diagram you had 150R which is about the right value. But you got more light when you changed it to 1K so that suggests you didn't have 150R in the first place.
what are the rules to the sensitivity of the phototransistor
Not sure what you mean here. The photo transistor will cause a current to flow from collector to emitter depending on the amount of light. That current affects the voltage on the output as the current flows through the resistor. So the higher the resistor the more change in voltage you will see for the same change in light levels.