Adult houseflies grow to 8–12 millimetres (0.3–0.5 in) long. The thorax is gray or sometimes even black, with four longitudinal dark lines on the back. The whole body is covered with hair-like projections. The females are slightly larger than the males, and have a much larger space between their red compound eyes (Wikipedia). I saw similar looking flies of three different sizes. This is the largest of the three.