Grimmith, there are two parts to your question.
1. Taillights are a direct swop in with the old filament bulbs - no resistors required.
2. Indicators: If you are using the old stock flasher relay then it requires a full load to operate i.e. 2 x 21Watt bulbs Left or Right sided. You may have noticed in the past that when an indicator bulb burns out the other bulb will not flash. This is because the load/current through the flasher relay has halved. The flasher relay has a bimetallic strip which heats up as the current passes through. When hot enough it switches the power off, on cooling it switches the power on again then starts to heat up and the cycle repeats. If a bulb burns out the current is halved and the bimetallic does not heat up enough.
When you change to LED the current drawn is very low compared to the old system so the old flasher reply just does not heat up. If you keep the old flasher relay you need to increase the current through it and this is done with a resistor as a dummy load. The resistor is acting like an old filament bulb and increases the current through the flasher relay. This resistor is not in series with the LEDs but in parallel.
What size resistor? The resistor replaces 2 x 21Watt bulbs. We can work out the effective resistance of a 21Watt bulb:
Voltage is approximately 14V
Watts = Volts x Amps
21W = 14V x 1.5Amps i.e. Each filament bulb draws approximately 1.5 Amp
(Note: If the LED unit is 3W then subtract 3 from 21 to give 1.3Amps and a resistance of 11 Ohms)
Volts = Amps x Resistance
or Resistance = Volts/Amps
Resistance = 14V/1.5Amp = 9.3 Ohms
Each LED indicator needs about a 10 Ohms resistor wired in parallel with it. If you use 4 x 10 Ohm resistor it gets a bit messy. Since for each side the front and back indicators are in parallel so you can just use a 5 Ohm resistor connected from the left indicator power wire to ground and another 5 Ohm connected from the right indicator power to ground.
Each 5 Ohm resistor takes the place of 42Watts (2 x 21Watts). I would use ceramic resistors rated at 100Watts (However, the indicators draw current 50% of time so 50W would do). These will warm up when in use so check they do not melt anything.
But....... If you replace your old flasher relay with a new LED flasher relay then you can forget about those resistor dummy loads.
Edit: See Team Junk's link immediately above this.