What bitrate is it now? How long is the song? What kind of music is it?
Note, if you use LAME to re-encode it, you can always do it with the LAME.exe file, and use the commandline. Or else use a program that lets you change LAME's quality setting to 2, or if possible, 0. It'll take longer to encode, but it'll give you as much quality as possible. Or use VBR, and fiddle with the quality and bitrate range settings to get it to the filesize you want.
Other ideal option to minimize quality loss: re-rip from the original CD, if that's an option, and encode that to the smaller file.