What is the largest galaxy, near or far, discovered to date? How large is it?
The largest galaxies that we know of in the present day are giant elliptical galaxies in the centres of clusters of galaxies. They are sometimes called cD galaxies (for giant diffuse galaxies) or BCGs (for brightest cluster galaxies). These galaxies are about 10 times more luminous that a typical galaxy (like the Milky Way) and about 100 times more massive. They can have diameters of more than 6 million light years (compare to about 100 thousand light years for the Milky Way). A good example is IC 1101, the central galaxy in the cluster Abell 2029.
cD galaxies are thought to grow by accreting any galaxy that comes anywhere near them, and in the centre of a cluster this can be a lot of galaxies over the age of the universe - so their mass can grow by a large factor. They usually have an area around them in which no smaller galaxies are found (as they've all been 'eaten') and a diffuse halo of gas and dust which is thought to be the debris of the smaller galaxies.
As a side note, this question gets more complicated the further away you look because of several competing effects. The most massive galaxies are the rarest galaxies, so you have to search larger and larger volumes of space to find them. And because of the finite speed of light, the further away you look, the further back in cosmic time you see. Since galaxies generally grow by gradually accreting gas and other galaxies, the further back in time you look, the smaller the galaxies are on average. However, since astronomers are basically measuring how bright galaxies are, we are biased to finding the brightest things, which are also the most massive things. So a lot of the times we find "surprisingly" massive things in the early Universe, which probably would be even more massive today if we could see them in their current form. But generally speaking present-day cD galaxies are the biggest!
This page was updated on Jul 18, 2015