Wrong paper size

I have a HP OfficeJet 150 Mobile All-in-one Printer/Scanner

Unfortunately, the scan size seems to be limited to letter size:

> scanimage -- help -d hpaio:/usb/Officejet_150_Mobile_L511?serial=MY2A9720DR
...
-y 0..277.707mm [277.707]
        Height of scan-area.
...

Trying to scan a A4 cuts off the bottom of the page. But as there is the limit, it is also not possible to set A4 as size:

> scanimage -x 210 -y 296 -d hpaio:/usb/Officejet_150_Mobile_L511?serial=MY2A9720DR
scanimage: setting of option --br-y failed (Invalid argument)

Some idea, how to fix/change it?

Thanks Ben