What size duvet cover should I buy for my comforter?

Match the cover size to the comforter, not the bed. A queen comforter goes in a queen cover (typically 90"x90"), and oversized king comforters need an oversized king cover (around 104"x90" or larger). Reviewers in r/Bedding repeatedly warn that buying one size up to 'leave room' results in a sloppy, shifting fit even with corner ties.