Are there studies that show that those with larger prefrontal cortexes make better decisions or do the studies just show that decision making occurs in the prefrontal cortex? — Hanover

Yes. I'm not an expert but my understanding is that there are a lot of evidence to believe that the ability to make rational decisions is heavily associated with the prefrontal cortex. The experiments I've read were on people who had seriously impaired or missing prefrontal cortex function though, not people with bigger/smaller normal prefrontal cortexes. The data I mentioned about how the girls who developed smaller prefrontal cortexes being less successful in life suggests that though.

Thinking generally takes place in the brain, but those with larger brains aren't statistically smarter than those with smaller brains, so I don't see why an enlarged prefrontal cortex should correlate with it working better. — Hanover

I think you're confusing how differences in the brain sizes between people doesn't necessarily dictate IQ, which is true. But that's because the differences are relatively small. In the animal kingdom a bigger brain is indicative of higher intelligence. Also better (bigger) functioning prefrontal cortex does not necessarily make you better at taking an IQ test, for example, but can provide you with higher impulse control which is still plenty useful.

The real message it would seem is that alcohol consumption affects a developing brain, so maybe hold off drinking until you're a little older. — Hanover

Girls definitely should be held off of drinking statistically, while boys might be improved by it even.