Audio files should be WAV, MP3, M4A, FLAC, AIFF, or Windows Media (WMA).
If you're sending a WAV, 16-bit, 44.1 kHz WAV is typical but pretty much anything works.
The maximum size that DistroKid will accept is 1gb. If you have a track that's larger than that, consider converting it to FLAC format before uploading to DistroKid. FLAC is a beautiful, lossless format (same audio quality as WAV) but the files are relatively small.
DistroKid has no limitation to the length in minutes that a track can be. However, albums cannot contain songs where the average track length is less than 60 seconds.
Please see here for some specific information about uploading via iOS: