I am going to use a voltage divider circuit where the motor is the variable resistance. The thing is I don't know what should be the value of the fixed resistor.

For the current measurement, I assume?

The specs give a stall current of 500mA (1/2 Amp) so you can use

Ohm's Law to calculate the approximate lowest motor resistance. (I'd use the "rated" 6V for that.)

You don't want to loose too much voltage across the resistor, but the voltage drop has to be high-enough to measure. So, choose a resistor that's no more than 10% of the (calculated) motor's lowest resistance. You can use a lower value resistor if you can get enough voltage & enough resolution.

(You can use the optional 1.1V ADC reference for more resolution at lower voltages.)

Please help since I already burned one resistor.

Power (Wattage, related to heat) is calculated as W = Voltage x Current. (For the resistor, that's the voltage across the resistor and the current through it.) Or, it can be calculated as I

^{2}R (which should be handy in this case) or V

^{2}/R. It's considered good practice to use a resistor rated for at least twice the dissipated power.