# For horizontal video
ffmpeg -i input_file_name -c:v libx264 -c:a copy -r 25 -b 3.5M -s 768x432 output_file_name

# For portrait video
ffmpeg -i input_file_name -c:v libx264 -c:a copy -r 25 -b 3.5M -s 432x768 output_file_name