On your TV remote (not the Roku remote), look to see if you have a "Picture size", "Aspect Ratio", or "Zoom" button (or similar language). May have to go into your TV settings and look for these things if not available via the TV remote button.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102