I recently came accross this problem and this is the solution I found.
- Go to the App Store and install the HP ePrint app
- When you start the app registration is optional. You will also need to grant permission for it to access your photos etc.
- Once done you can go to photos via this app and choose the photo you want to print.
- There is a toolbar above the photo where you can change preferences before printing and one of these is paper size – in my case I changed it from A5 to A4.