It is then explained that the "ball" is about a mile in diameter. I don't remember anything about the plot – just this opening gag line. It was in an anthology I read as a kid. Possibly by a well-known author. Of course the terms trans-human and post-human aren't used, but the concept is there.
There is a line like this in Legends from the End of Time by Michael Moorcock.
My Lady Charlotina's Ball must have been at least a mile in circumference, set against the soft tones of a summer twilight, red-gold and transparent so that, as one approached, the guests who had already arrived could be seen standing upon the inner wall, clad in creations extravagant even at the End of Time.
The Ball is literally a ball:
The Ball itself was inclined to roll a little, but those inside it were undisturbed; their footing was firm, thanks to My Lady Charlotina's artistry. The Ball was entered by means of a number of sphincterish openings, placed more or less at random in its outer wall. At the very centre of the Ball, on a floating platform, sat an orchestra comprised of the choicest musicians, out of a myriad of ages and planets, from My Lady's great menagerie (she specialized, currently, in artists).
This is the fourth book in Moorcock's Dancers at the End of Time series, and that fits the description of post humans since the protagonists are immortal decadents with nothing better to do than mess around and indulge their fantasies.