How can I reduce a video's size with ffmpeg?

How can I use [ffmpeg](https://ffmpeg.org/) to reduce the size of a video by lowering the quality (as minimally as possible, naturally, because I need it to run on a mobile device that doesn't have much available space)? I forgot to mention that when the video can use subtitles (*.srt or *.sub), I'd like to convert them too to fit the parameters of the converted video file.