Re: Need to drop 12v to 6v. What resistor to use..
The 'use a regulator instead' advice is right. FWIW, I too recommend the 7806, being fixed, it's probably easiest to use. The part and a cap or two and you're good to go.
It is not at all safe to plan to use a simple resistor. The light is not a simple 'six volt, 300 mA' load. As the gas is cold, the tube (the load for the inverter in the light) changes dramatically as it goes through it's start up cycle. Current drawn changes a lot, typically. A high impedance 12 Volt source (what you have with a series resistor) is going to deliver wildly changing voltages, and may indeed dip far under six (stalling the cycle) as well as being high as the tube lights. In this (lighting) time, the very high impedance of the lamp tube is 'reflected back' into the driver (where the peaks can be many times higher, destroying it by overvoltage).
A regulator fixes that.
Doug Owen