The chart that Ninjakitty posted is helpful. I found this site to be even more helpful, as it basically figures it all out for you: Circuit Wizard - Blue Sea Systems
Just input the voltage (12v, 24v, etc - whatever you're using), the load in amps (max that you want able to go down the wire), length (to and from the load, so - total circuit). The allowable voltage drop should be 3% for critical loads (inverter, etc) and 10% for non-critical loads (lighting, fans, etc). Punch in the duration of time you expect to use it in one go (for example, lights might be on for 4 hours, a fan might be on for 8 hours, etc). Then click Calculate.
It'll tell you what gauge wire to use and the maximum capacity of the wire. As long as you stay within this guideline, you can run whatever you want. And, remember - you can always go with bigger wire, but you never want to go smaller.
Hope that helps! I learned most of that from this site, which may or may not be helpful: faroutride.com/electrical-system