To Convert an MP3 to a Smaller File, you have to Launch iTunes. You need to Click "Edit" at top of the screen and click "Preferences". You must Click the "Advanced" tab and click the "Importing" tab below it. You should Confirm that the "Import Using" drop-down menu has "MP3 Encoder" selected. If it doesn't, you may change it.
After that, Click the drop-down menu next to "Setting" and select "Custom." Then, Select a bit rate for your converted MP3 files. You can clicking the "Channels" drop-down menu and selecting "Mono" and choose the bitrate at 64 kbps. Next, Right click on the MP3 file that you would like to reduce the size of and click "Convert Selection to MP3".