#30 wire is plenty, anything bigger is just a bonus. I wire all my LED lighting upgrades in locomotives with #30 wire, easy to hide.
The series resistors for a bank of three are 150 ohms 1/4 watt. These are really load balancing resistors in this case, as every string of three won't have exactly the same characteristics. The resistors allow all of the LED's to run at the same intensity.