Giant molecular clouds (GMCs) are massive, dense regions of interstellar gas and dust where star formation occurs. They are typically found in the spiral arms of galaxies and can range in size from a few parsecs to several hundred parsecs. A parsec is a unit of distance equal to about 3.26 light-years.

The largest known GMC is the Sagittarius B2 cloud, which is located in the Milky Way galaxy and has a diameter of about 150 parsecs. This cloud is thought to contain enough mass to form several million stars.

The average size of a GMC is about 50 parsecs, and they typically have masses of about 100,000 solar masses. Solar mass is a unit of mass equal to the mass of the Sun.

GMCs are important sites of star formation because they provide the necessary conditions for stars to form. The gas and dust in a GMC is very cold and dense, which allows it to collapse under its own gravity. As the cloud collapses, it fragments into smaller clumps, each of which can eventually form a star.