Two astrophysicists have estimated the largest possible thing in the universe for there is an upward potential limit due to the age of the universe.

A black hole could conceivably have grown to the massive size of a hundred thousand tredagrams*, or 50 Billion suns in size.

Based on this self-regulating maximum rate, scientists at the Harvard-Smithsonian Center for Astrophysics, Massachusetts, and the European Southern Observatory, Chile, have calculated an upper limit for these mega-mammoth masses.  Fifty billion suns, that's 100 000 000 000 000 000 000 000 000 000 000 000 000 000 kg, otherwise known as "ridiculously stupidly big" and triple the size of the largest observed black hole, OJ 287.

That's a big twinky.

New Scientist article.

*note:  As an interesting aside, a tredagram is not even an officially recognized SI prefix of measurement in the metric system.  Merely a proposed prefix.  This number is sooooo large that we don't even have a clearly defined unit to express it.  Here is a pdf, wherein the the new unit of measure is discussed.