[Federico]

Let's say you have an mp3 file at a bit rate of 128kb/s and a sample rate
of
44.1k samples/second, and you want to cut the size of the file in half.

Cut the song in half and keep the first part. :-)
For example I see that on myspace.com-music people tend to listen to the
first minute of the songs.
If it's for a marketing thing maybe you don't need the second half of the
audio track.
F.