A typical globular cluster is a few hundred light-years across. Emission nebulae are usually the sites of recent and ongoing star formation. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.
Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind.
Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. Our Sun will probably produce a planetary nebula in about 5 billion years. A typical planetary nebula is less than one light-year across. The Hubble Space Telescope has zoomed in on the center of the Crab Nebula, producing an image with unsurpassed detail of the convoluted celestial marvel, astronomers said Thursday.