Many people were noticing this change since a fortnight. However, Google officially confirmed the news on 1st December 2017, late Friday. It has increased to a maximum character length of 320 characters.
What is Snippet?
Basically, a featured snippet is a synopsis of a solution to a user’s query. It is displayed on top of Google search results. Snippet text is extracted from a webpage, and includes page’s URL and title.
Danny specifically mentioned “don’t go expanding your meta description tags.” As per him, having longer snippets are “more a dynamic process” Google automatically makes based on the content on that page and your query
Note that yet Google hasn’t come up with any statement regarding changed / changing norms of meta descriptions.
Danny referenced a previous tweet from John Mueller of Google who said not to actually make any amendments in existing meta descriptions:
The impact of longer snippets is yet to be seen. Would you like to add anything here? We’d love to hear from you!