Rusty,
MP3 is a "lossy" format, which means that compression causes a degradation (loss) of fidelity, but yes, you can reduce the size of an MP3 by encoding at a lower bitrate. That will also reduce the size of the actual file. For Girls Rock Radio, I specify that artists must upload music submissions at a bitrate of 256k minimum, 320k preferred. The station streams at 128k, but to maintain the best fidelity, I have to start above 128 so the stream audio processors and encoders don't noticeably affect the quality of the sound.
For you, a program like Audacity (free - http://audacity.sourceforge.net/) could be used to load your master audio file, then specify a size-friendly bitrate to save in MP3 format.
256k + = CD Quality
128k = Near CD Quality (but better than FM quality)
96k = FM Quality
56K and below = starts to loose fidelity quickly with 28K sounding about like AM radio.
Experiment with these to find a good compromise between audio quality and file size for upload.
Does this help?