According to Google, the resizable sidebar sections are rolling out over the next few days to Google Workspace Essentials, Business Starter, Business Standard, Business Plus, Enterprise Essentials, Enterprise Standard, and Enterprise Plus, as well as G Suite Basic, Business, Education, Enterprise for Education, and Nonprofits customers.

Since normal Gmail accounts don't have access to Chat and Rooms quite yet, this won't affect those users — although that may change in the near future as Chat starts to fully replace Hangouts