You are confusing filesize and length. Making them lower bitrate won't make them fit on a CD

A cd will fit 80 minutes. If the CD is longer than 80 minutes it is probably ripped from a DVD or something like that.

If the length is 1 file with like 150 minutes of mp3 then you can cut the file instead

This tool is very good

Download Mp3Cutter4 (Link)