To be precise, I should have said common value. While a 1/4 watt rating infers size, the 680 ohm 1/4 watt resistors are the most common value and size that will safely sink 12VDC. 1/2 watters are availible but getting less and less common as electronic devices become more efficient. Thirty years ago, just the opposite was true and 1/2 watters were your most common size and readily available in more common values. I use a 5x2 array in my eyepiece and finder heaters. I alternate the wiring on one end then I can switch in five for a bit more than 1 watt and if that is not enough, switch in the remaining five for about 2 1/4 watts. If that is not enough, it is to wet for viewing. I use Nichrome resistor wire for the correctors.

Good Luck,
-Rick