Do a search here as I know this has been discussed before and there were some good links to websites posted up about gauge sizing etc.
A quick Google search netted this: http://www.the12volt.com/info/recwirsz.asp
The answer depends on how many total amps you plan on running off the block (looks like the max you could do is 180) plus the distance from the battery. You want to make sure you are getting the full power needed to the block and not losing a bunch in heat/resistance in the cable do it being too small. I ran 2ga power and ground wire from my battery in the LX to the fuse block under my PS seat. For you that might be overkill and either 4ga or 8ga would be plenty robust.
I wanted to exceed the requirement and only do things once so I went with the big and forget about it plan
- So, if you figured your max is 180 amps and the block will be 8' or less from the battery I'd go with 4ga cable and forget about it. 8ga would limit you to about 1/2 of the max the block will pull at that distance, theoretically. Of course you'll probably never have all of the light and the CB and the HAM etc. all running at the same time so...