Two hundred and eighty characters

A message is 280 characters at most. The server refuses anything longer, and the app shows a counter that only appears for the last sixty.

It isn't a bandwidth constraint. We could accept ten thousand characters without it costing anything.

What the limit prevents

It prevents the monologue. On a bench nobody talks for six minutes straight: the others would leave. A short limit reproduces the social constraint that screens removed.

It also prevents the long argument. Online rows need room to quote, rephrase and rebut point by point. In 280 characters you can only be blunt, and blunt runs out of steam fast.

280 rather than 140 because people write long, and too tight a limit pushes you into telegram style. Enough for a whole thought, not enough for a paragraph.