It wasn't arbitrary. Twitter started out with an SMS interface, and kept it for several years. SMS are 160 characters max [1], and some characters are needed fo

It wasn't arbitrary. Twitter started out with an SMS interface, and kept it for several years. SMS are 160 characters max [1], and some characters are needed for the protocol, so the payload was limited to 140 chars. [1] Note: Modern SMS apps support a protocol that sends larger messages as a stream of multiple interlinked SMS. This is transparent to the user, but at the time of Twitter's SMS interface this was not common.