Twitter’s original maximum length for a direct message was 140 characters.
The limit reflected Twitter’s early relationship with SMS text messaging. Public tweets were also limited to 140 characters, allowing a message to fit within the technical constraints of a mobile text message. Direct messages therefore began with the same compact limit, even though they were private rather than public.
Twitter increased the direct-message limit to 10,000 characters in 2015, separating private conversations from the short format of public tweets. That change is often confused with the later expansion of public tweets to 280 characters in 2017. The two changes affected different parts of the service: direct messages became much longer in 2015, while the public-post limit remained 140 characters until 2017. The original number is therefore a historical product specification, not the current limit for private messaging.