Straight to the point this time. Let’s say we want to resize an 8K video to 4K or perhaps a 4k video to HD.
To downscale while keeping the aspect ratio use:
ffmpeg -i input.mp4 -vf scale=1920:-1 output.mp4
To set the resolution regardless of aspect ratio (might stretch the video and make it ugly) use:
ffmpeg -i input.mp4 -vf scale=1920:1080 output.mp4