I have not tested but AVAssetExportSession can help you with this.
MPMediaItem *curItem = musicPlayer.nowPlayingItem;
NSURL *url = [curItem valueForProperty: MPMediaItemPropertyAssetURL];
AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL: url options:nil];
AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset: songAsset
presetName: AVAssetExportPresetPassthrough];
exporter.estimatedOutputFileLength will give you size in bytes.