Does resizing hurt quality?

Scaling down is generally clean — detail is reduced but artifacts are minimal at these presets. Scaling up adds pixels without adding detail, so the tool warns you when the source is already smaller than the target.

What happens to the audio?

It's copied across untouched — only the video stream is rescaled.

Is the file uploaded?

No — rescaling runs on your device.

Resize a Video Online — Free Resolution Changer