Please check the below code.
Calculate Available Internal Storage memory
StatFs stat = new StatFs(Environment.getDataDirectory().getPath());
long bytesAvailable = (long)stat.getFreeBlocks() * (long)stat.getBlockSize();
long megAvailable = bytesAvailable / 1048576;
Calculate Total Internal Storage memory
StatFs stat = new StatFs(Environment.getDataDirectory().getPath());
long bytesAvailable = (long)stat.getBlockSize() *(long)stat.getBlockCount();
long megAvailable = bytesAvailable / 1048576;
Calculate Available External Storage memory
StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath());
long bytesAvailable = (long)stat.getFreeBlocks() * (long)stat.getBlockSize();
long megAvailable = bytesAvailable / 1048576;
Calculate Total External Storage memory
StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath());
long bytesAvailable = (long)stat.getBlockSize() *(long)stat.getBlockCount();
long megAvailable = bytesAvailable / 1048576;
Note : First of all you need to calculate avilable internal memory and show to user where he/she wants to download file.