Well you can change the bitrate of a track, the lower the bitrate the less space it takes. It really depends on what your mp3 player can play (MP3/WMA/OGG ect.) and the type of sound you like. On my Dell DJ I keep most of my music at MP3@128KBPS, it's what I find most comfortable. Sound opinions are biased since not everyone likes the same quality in sound, some people can't stand anything less than 192KBPS some People are fine at 68KBPS, it really depends on you.
Just google "Mp3 Bitrate Converter" or "Bitrate Changer", there are tons of app's that do the job, I use Acoustica MP3 Audio Mixer.