you need a software to edit MP3 if your cell let you use mp3 as a ringtone, for example MOTOROLA PHONE TOOLS is a good way to do it, just create your own ringtones and do not worry for the file size.
At: JC
How do i change a file size smaller so it would fit on my cell phone{ringtone}?
try winrar it might compromise it enough but to open it i dunno
How do i change a file size smaller so it would fit on my cell phone{ringtone}?
get a program EG.Nokia PC Suite