I don't know what size is too big or if quality is an issue to the person you are sending it to but...

Unless the .wma file is lossless, don't convert it as you will lose quality and add artifacts that will probably make the audio sound weird. Hopefully you have the original song in .wav and can perform a conversion again. If your wma file is 160, try a smaller (more compression) number such as 128 or 64. You can also try converting to a mono file which should take half as much space.

There are web sites which let you upload a large file to share and e-mail providers which allow fairly large uploads. But first try a deeper compresion and see if that works.

R