Galaxies are vast collections of stars, planets, nebulae and other objects that are surrounded by an interstellar medium and held together by gravity.
They do not have a definite boundary by nature, and are characterized with gradually decreasing stellar density as a function of increasing distance from its center.
Because of this, measuring the sizes of galaxies can often be difficult and have a wide range of results depending on the sensitivity of the detection equipment and the methodology being used.
This list uses the mean cosmological parameters of the Lambda-CDM model based on results from the 2015 Planck collaboration, where H0 = 67.74 km/s/Mpc, ΩΛ = 0.6911, and Ωm = 0.3089.
The reference to those sizes plus further additional details can be accessed by clicking the link for the NASA/IPAC Extragalactic Database (NED) on the right-hand side of the table.Listed below are some notable galaxies under 700,000 light-years in diameter, for the purpose of comparison.