How do I select the Correct Diodes and Resistors for my LEDs?
For 12-13W and a 12V battery, that equates to roughly 1A, so you could do with at least 2A diodes. This means anything above this is okay, 6A is maybe a little overkill but will work.  

Follow the diagram in the thread and you should get dim/fully lit on the turn signal, the 470&Omega; resistor should be a 0.5W or above.

The 470&Omega; determines how dim the light is when the blinker is off, and will only pass around 25mA maximum (see Ohm's Law). The diode does pass the fully lit current of around 12W / 12V = 1A, so it ideally needs to be rated for at least double.