How to change Bitrate and other settings for MP3/OGG/WAV/FLAC?

Click the gear button to open Options

Go to 'Files' tab

Then, click MP3, OGG, WAV, FLAC label to open the related settings.

For minimizing the file size of the output file

Save recordings as MP3
Sample rate: 8000 Hz
Channels: Mono
Mode: Normal
Bitrate: 8 kbps