How do I print iPhone photos as 4x6 instead of 8.5x11?

I would suggest duplicating the photo that you want to print, then cropping it to a 4x6 aspect ratio using Photos.

Duplicate and copy photos and videos on iPhone - Apple Support

Crop, rotate, flip, or straighten photos and videos on iPhone - Apple Support

I have not tried this on an iPhone. I'm using iCloud Photos – and find my Mac to be the more convenient device for editing. But the second Support article indicate that the Photos app on the iPhone can crop a photo to a standard preset aspect ratio.

In the Mac version of Photos, when you crop to a preset aspect ratio, you can choose which parts of the picture get cut off, and which are preserved. Hopefully the same is true for the iPhone version.

"How do I print iPhone photos as 4x6 instead of 8.5x11?: How can I print photos as 4x6 instead of 8.5x11. I have an iphone 12 Pro Max."

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Resizing Photos:

What's the model of your Printer? Which app are you using?

Ruler:

If an option for the app, you can select page size in Pages Go here: Show the Ruler in Pages on iPhone - Apple Support

HP:

If HP, set up AirPint and then change settings. Link form HP: HP printer setup (Apple AirPrint for iPhone, iPad) - HP Support