The recommended and maximum file size and bitrate for audio files is as follows:
File size: < 2 mb suggested, < 2.2 mb max
Bitrate: < 128 kbps suggested, < 198 kbps max
Suggested numbers are to prevent you from running into issues with maximum size limits (where the limit refers to the maximum size you can use before losing out on significant scale).
Note: Some major publishers allow higher file sizes and bitrates.