The size of the files when converted to mp3 can vary in size depending upon your quality settings. When they are mono, stereo, joint stereo, & their bit rate: variable or 48 to 320 kbps, & their quality setting, Sample Rate & type of codec used to do these conversions. There's more than one type of mp3. And there's also a newer format known as m4a/m4p which enables them to be compressed even further in size than mp3 whilst maintaining reasonable sound quality (however this latter format is not recognised by all mp3 players & thus should be used with some caution.)