http://www.astronomy.com/~/link.aspx...9-17f391112488

"Astronomers have crowned spiral galaxy NGC 6872 the largest-known
spiral based on data that shows it spans more than 522,000 light-years."

This galaxy has been disrupted by its neighbouring galaxy, IC 4970,
which is also in the picture and it is "only" 100,000 light-years
across. Compared to NGC 6872, IC 4970, looks like the Large Magellanic
Cloud compared to the Milky Way, i.e. it looks like a dwarf galaxy.
However, this dwarf is as large as the Milky Way itself, in absolute
terms! It's almost like as if we have a direct comparison of the Milky
Way to NGC 6872 in the same picture, so we don't have to imagine how big
the Milky Way would look next to this galaxy, we have it already in
place in this picture.

Yousuf Khan