Yeah, Distortion88 is right. Wave is the biggest, but has the best quality since it isn't compressed what so ever (though at a high enough MP3 bitrate, the quality isn't noticable). That is why the file size is bigger.
Where did you get your information from that MP3 was bigger than WAV?
And yeah, if you use a program like:
You'll be able to use that which will automatically convert MP3 to the desired bitrate to get the size you want. It should also convert WAV back into MP3.
I suggest to keep the quality/vs size on an MP3, you don't drop the bitrate any lower than 128kbps. Any lower than that, and you'll be able to tell the quality has significantly dropped.
Keeping at that bitrate should provide about 1MB a minute for a song.
So a 3 minute song would be 3MB.
I hope this helps.
The same song in WAV format, could easily exceed 20-30MB.