It starts out at 291MB per track, which is of course the exact size of “Inamorata and Narration by Conrad Roberts” from side 4 of Miles Davis’ Live-Evil (assuming we’re talking 16/44.1, which we always are). Once you’ve made a few sales through Bandcamp (totaling $20 USD or more), we increase the max per-track upload size to 600 megs. There is no limit on the size of albums.