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
Bitrate: 8 kbps