When you are AirPrinting from an Apple mobile device, there are no hidden preferences that you can modify. It is really just print as is.
I would recommend downloading the HP ePrint Home & Biz app or the HP Printer Control app from the Apple App Store. Either of these apps will give you multiple settings that you can modify, and picture sizing options.
If I have solved your issue, please feel free to provide kudos and make sure you mark this thread as solution provided!
Although I work for HP, my posts and replies are my own opinion and not those of HP.
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