Audiobook files vary in size depending on their content. Each Harry Potter digital audiobook will be between 200MB and 1GB in size.
Your books can be found at https://bookshelf.pottermore.com.
To download an audio book directly to your device, you will need unzipping software (WinZip/7Zip), on your device, plenty of storage space and a strong internet connection. The file may take some time to download due to the file size.
Alternatively, you can download to your computer and add to your device manually.
If you are still having problems, please Contact Us.