Automatic switch for shop vacuum
I think I understand the intent now. 

Sizing the resistor will depend on how much voltage out of it you will need. 

For example, if your mains are 120V then a 1500W motor will draw 12.5V RMS. Suppose you want 6V output at full load. Then simply use ohms laws V=I*R (6/12.5) the answer would be 0.48 ohms. The second calculation that would be needed is the wattage. The calculation would be W=I^2*R (12.5^2*0.48). This gives you 75 watts. This is like a large light bulb and will be a hefty resistor. 

The drawback is that this is with the motor at its absolute max consumption. If the motor is running at 50% output the voltage will also only be 50%.