Resize a video to 1280x720, the 720p frame that clears most attachment and upload limits.
Against 1080p
720p carries 44% of the pixels of 1080p at the same 16:9 shape. Both sides divide by exactly 1.5, so the scaling is clean and no rounding is involved.
At the same quality setting the file lands at roughly half the size. That is the point of this size and the only reason to choose it.
Frame size against bitrate
There are two ways to make a video smaller: fewer pixels, or fewer bits per pixel. They are not equivalent.
At a fixed target size, a smaller frame usually looks sharper than the full frame squeezed harder, because the encoder has more bits for each pixel it keeps. Below a certain bitrate, squeezing 1080p produces blocking in motion while 720p at the same total size stays clean.
If the requirement is a file size rather than a frame size, the video compressor takes the target directly and works the bitrate out.
Where 720p shows
Text, user interfaces, screen recordings, and fine texture. Anything watched on a laptop or a television at full size.
Where it does not show: handheld footage watched on a phone, background loops, and anything a feed platform will re-encode on arrival anyway.