A wonderful article in Nautilus titled Can a Living Creature Be as Big as a Galaxy? is informative to the max, complete with a terrific set of illustrations, courtesy John Hendrix.
The size of things in our universe runs all the way from the tiny 10-19 meter scale that characterizes quark interactions, to the cosmic horizon 1026 meters away. In these 45 possible orders of magnitude, life, as far as we know it, is confined to a relatively tiny bracket of just over nine orders of magnitude, roughly in the middle of the universal range: Bacteria and viruses can measure less than a micron, or 10-6 meters, and the height of the largest trees reaches roughly 100 meters. The honey fungus that lives under the Blue Mountains in Oregon, and is arguably a single living organism, is about 4 kilometers across. When it comes to known sentient life, the range in scale is even smaller, at about three orders of magnitude.
Addendum: What about on the supersize end of the spectrum? William S. Burroughs, in his novel The Ticket That Exploded, imagined that beneath a planetary surface, lies “a vast mineral consciousness near absolute zero thinking in slow formations of crystal.” The astronomer Fred Hoyle wrote dramatically and convincingly of a sentient hyper-intelligent “Black Cloud,” comparable to Earth-sun distance. His idea presaged the concept of Dyson spheres, massive structures that completely surround a star and capture most of its energy. It is also supported by calculations that my colleague Fred Adams and I are performing, that indicate that the most effective information-processing structures in the current-day galaxy might be catalyzed within the sooty winds ejected by dying red giant stars. For a few tens of thousands of years, dust-shrouded red giants provide enough energy, a large enough entropy gradient, and enough raw material to potentially out-compute the biospheres of a billion Earth-like planets.
Food for thought, eh? :)