there are three ways of encoding :-
1)Mono:- a)constat bitrate -you can set from 18 to 56 kbps
b) average bitrate - you can set from 48 to 96 kbps
2) Stereo - a) CBR - 32-96 kbps
b) VBR - 48 to 96
3) LC-stereo - a) CBR - 18-56 kbps
b)VBR - 48-96 kbps
You can also set the speed of the encoding which also determines the quality
Note : - "The above mentioned info isfrom Jet Audios Mp3pro encoder"
Generally mp3 pro encoder can hold a standard time such as 5 minutes song in 560 kb 's , yes almost half mb when encoded at LC stereo at 18 kbps
now i think you have got the info right?