Not only do nebulae come in various sizes, but also various shapes and colors.
If I recall my University astrophysics correctly, to the naked eye they are much less colourful. The colours we see in pictures are 'real' in a sense, but not the conventional sense most would understand by the word. They show variation in the wavelength of light, and we enhance those visually to colours which humans can see.
indeed, they are actually radiate by the reflected star light, the photos are all taken by the telescopes thou, not sure if they do the enhancing themselves or down at Earth after they get the pictures.
For many the colour comes from long exposures. Others are false colour enhanced.