Davinci Resolve 16: Need to restore timeline to original aspect ratio
It's hard to know what's going on without knowing your project settings or what kind of footage you're using.  But the relevant settings are under Project Settings->Master Settings->Timeline resolution, Project settings->Master Settings->Pixel aspect ratio, and Project Settings->Image Scaling->Input Scaling->Mismatched resolution files.  Unless you're editing old DV footage or unless you used an anamorphic lens (you would know), Pixel aspect ratio should be set to Square, and Mismatched resolution files should be anything except "Stretch Frame to all Corners."