Scaling video with FFmpeg can be done with defined width and height like:
ffmpeg -i input.mp4 -vf scale=1920x1080 output_1080p.mp4
Assuming that my video is 16:9 ratio like 1280×720.
However if you didn’t have a conventional ratio like 16:9 and didn’t know the upscaled and down scaled sizes for your videos ratio there is a simple command to do that work for you.
To upscale double of what your original video is
ffmpeg -i input.mp4 -vf scale=iw*2:ih*2 output_double.mp4
iw*2is width x 2,
ih*2 means height x 2. You can also times by 3, 4 etc for a bigger size.
Down scaling whilst respecting the ratio is done like:
ffmpeg -i input.mp4 -vf scale=iw*.5:ih*.5 output_half.mp4
This is multiplying by .5 to get half. Another method is to divide by 2
These scaling methods are quick and do auto calculation on width x height based on the original video ratio.