To set a bitrate for a video encode in FFmpeg use
-b:v which means bitrate for video, then specify the bitrate value. This can be k for kBits or m for megabits,
-b:v 2M is 2 Megabits.
An example for a 3 Megabit video bitrate is
ffmpeg -i in.mp4 -b:v 3M out.mp4
As an extra you can also specify the audio bitrate using
A working example with this in use is
ffmpeg -i in.mp4 -b:v 2M -b:a 124K out.mp4
Using this method is a one pass encode which is not efficient, making using
-crf a better approach (see here).