A human egg cell is about .1 millimeters, or .004 inches, in diameter. It is one of the largest cells in the human body and is just barely big enough to be seen without the aid of a microscope.
Human eggs are between 40 and 50 times larger than sperm cells. This means that fertilization in humans involves the union of one of the smallest cells with in the human body and one of the largest. Why there is such a large size disparity between the two is unknown; however, it can be due to smaller cells being easier to produce in huge masses, thus increasing the chances of fertilization, while larger cells are more likely to produce a viable zygote once fertilized. The size disparity therefore increases the chances of producing offspring for both parents.