My Premiere Plus (& 2016 Sony 1080P TV) did this recently after I exited the 'nowhere TV' app. Everything was zoomed in & I could barely read / maneuver thru the menu choices. I did manage to get to the Roku RESTART setting, which after restarting, everything was back to normal, and Premiere Plus hasn't 'zoomed in since', although I did delete the 'nowhere TV' app.
You might try to get to RESTART in Settings. OR unplug your Roku, wait a few minutes, and plug it back in. Might work or might not?