There is a good chart of wire recommendations at
http://www.redrival.com/ixlnxtc24/wiresize.htm

The lengths and gauges are calculate for 0.5V drop.  If you know any
electricians they pretty much all have a little book with lists of voltage
drops per foot, per current draw (Amps, you can caculate this roughly if you
know the wattage of the bulbs.  (Wattage) divided by 12 = Amperage.  Not
exact, but good enough for this application).

Here is the reason why this matters (Again, not 100% accurate, but good
enough for government work ;):

Let's talk about a 1Volt drop.

*If you have a pair of 60 watt bulbs running it would be a total of 120 watt
potential draw.

* Power (Watts) = Amperage x Voltage
  120 Watts = A x 12Volts
  A = 10 amps  (How much current they would draw at 12volts)

* Now calculate the resistance of the bulbs so we can see how Voltage effect
this power output.
  Voltage = Amperage x Resistance
  12 Volts = 10 amps x R
  R = (12) divided by (10) = 1.2 ohms

* How much current would these bulbs draw if they only received 11 Volts (We
can do this because the resistance of the bulbs will not vary with voltage)?
  Voltage = Amperage x Resistance
  11 Volts = A x 1.2 ohms
  A = 9.17 amps

* Finally, how much power (watts) are our two 60 watt bulbs putting out now?
  Power = Amperage x Voltage
  Power = 9.17 amps x 11 volts
  Power = 100.87 watts

As you can see, your 60 watt bulbs would be functioning like a pair of 50
watt bulbs.

Mike B