Android.os.StatFs

private String formatSize(long size) {
    return Formatter.formatFileSize(this, size);
}
// ...
{
File path = Environment.getExternalStorageDirectory();
StatFs stat = new StatFs(path.getPath());

long blockSize = stat.getBlockSize();
long totalBlocks = stat.getBlockCount();
long availableBlocks = stat.getAvailableBlocks();

// SdSize
formatSize(totalBlocks * blockSize);
// mSdAvail
formatSize(availableBlocks * blockSize) + readOnly;
}