ffmpeg - Video - Resize - Maintain aspect ratio

-vf scale=1920:-1
-vf scale=-1:1080
-vf scale=1920:-2
-vf scale=-2:1080

NOTE:

-1 for the width or height will keep it in ratio to the other specified dimension.
-2 will keep it in ratio to the other specified dimension, but, to ensure it is divisible by 2 (a requirement for certain encodings such as yuv420p) the width or height will be adjusted if necessary.

Example

ffmpeg -i input.mp4 -vf "scale=1920:-1" -c:a copy output.mp4

NOTE: The -1 tells ffmpeg to automatically calculate the height while maintaining the aspect ratio after sizing to the new width.

-i input.mp4: Specifies the input video file.
-vf “scale=1920:-1”: Applies the scale filter to resize the video width to 1920 pixels, and the video height is calculated to ensure the aspect ratio is maintained.
-c:a copy: Copies the media streams without re-encoding.
output.mp4: Name of the output file.