A study funded by the U.S. National Institutes of Health, the Canadian Institutes of Health Research and the Albert Einstein/Irving Zucker Chair in Neuroscience at McMaster University was done in order to examine the correlation between brain size and intelligence.

The results came back positive for the importance of the brain size.
Brains of 100 people were examined, among which 58 women and 42 men who all lived and died from cancer. While still healthy, they had agreed take the intelligence tests as well as donate their brains to science after death.

This study is different from the others because it is looking at the size of the brain directly and not through imaging. The study looked specifically at the size of the cerebrum, the thinking part of the brain, and found a correlation with verbal and spatial intelligence in women.
It is suspected, and further work will show, that in men it isn't the size of the overall brain but the structure that is correlated with how well they did on all tests. It is believed that it might be the size or the anatomical microscopic structure of the region in the brain that is important for that kind of thinking.

The study also found that brain size decreased in men over the years, which was not the case in women.
Albert Einstein’s overall brain was a normal size but the inferior parietal region in the brain, which is related to mathematical reasoning, was 15% wider on both sides than normal.