IMHO - the only true measure for "useable light" is a lux meter (1 lux is the goal, the amount of light required to comfortably read a newspaper) at a *useable distance..
*useable distance for me is at LEAST 50 m preferably 80 m , Led's are insanely bright to look at - it's how well they illuminate that is the question....
I specifically didn't mention colour temperature - 3300 - 4200 Kelvin is my preference to reduce visual fatigue.
To answer the wiring question, if you have an AMMETER then you can pick the right size wiring to minimise loss due to internal resistance (you lose energy directly from the wire as heat, wire simply gets hot when overloaded) - if this isn't an option then grab yourself some 6mm diameter wire - nothing your light bar can draw will kill that!
Here's a link for you to look up the right wire size
http://www.powerstream.com/Wire_Size.htm