Fireside recommends the following MP3 export settings, which are considered to be the industry standard:
mono, 64-96kb/s, 44.1k sample rate
These settings represent the perfect compromise between quality and file size, and is a widely accepted standard. Even multi-hour-long episodes with several hosts, sound effects, and music will come in below the 100MB mark using these settings.
For more information, please review Apple‘s file export guidelines.