Hi David,
Assuming you don't want to split the file at all, what you probably want to do is re-encode the file with a lower bitrate. This will reduce the quality of the audio, but will also reduce the file size. When you choose to save the file as .mp3 you will be given mp3 encoding options. I would choose Variable Bit Rate (VBR), as it results in better quality for size, and then experiment with different settings to see what file sizes you get. Make sure you save to a different file so you don't overwrite your original file.
Alex