You should say the size in minutes if you're going to burn an Audio CD, and the size in MB if you're going to create a data CD.
If you want to divide the MP3 file physically, you can use MP3DirectCut, MPEG Audio Scissors, etc.
If you only want to cut it for burning, you can use the Track Editor in Feurio: click the position you want to cut and press "Set end position" (Action menu).
If you use Nero, click properties of the track, and select the "indexes, limits, split".
The original file will not be modified with Feurio or Nero.