This Page should answer some of your questions around what resistors to use as well as explain the math behind it.
Amps = Volts / Resistance
Resistor Ohms = (Alternator Voltage / 2.0 Amps) - Injector Resistance
For example:
Resistor Ohms = (14.0 Volts / 2.0 Amps) - 1.2 Ohms
=> Resistor Ohms = 7.0 - 1.2 = 5.8 Ohms
Use the calculator below. Enter your injector resistance in ohms, your hold current in amps, as well as your injector impedance in ohms, in the form below and click the "Compute resistor" button. The Yellow Box will show your results.
Some people say to use a 10Watt Resistors, some say to use a 25W resistor, but where are they getting these values from? More than likely, they "heard" it from someone and are just repeating it. To find the correct wattage of the resistors you will need to do the following math:
Volts / Ohm = Amps
Watt = Amps * Volts
Example:
If you had a system that was 12Volts and a injector at 6ohm this would be 2Amps (12 / 6 = 2).
These 2Amps at 12 volts = 24Watts. This would mean that you should use a 25Watt Resistor. Easy right? Try it out to see if your friend told you the right watt resistor to use.