This tool changes resolution rather than duration. Fit keeps every part of the picture visible and adds colored borders when the source and output shapes differ. Fill covers the new frame and removes equal areas from opposite edges. Stretch uses every output pixel but can make people and objects look wider or taller.
The browser loads a WebAssembly build of FFmpeg only when you start the export. It scales frames with Lanczos resampling, writes H.264 video with AAC audio, and removes subtitles, chapters, attachments, and source metadata. The practical file limit is 100 MB on mobile or lower-memory devices, 250 MB on a typical desktop, and 500 MB on high-memory desktops.