You can set the paper size to "Custom" using the Embedded Web Server (EWS).
Custom is the likely the last entry in the paper size list.
EWS – Browser – Tray and Paper Management
Open your favorite browser > type / enter Printer IP address into URL / Address bar
Tab Settings> Preferences> Tray and Paper Management
If applicable, check the Advanced section (at the bottom of the Tray and Paper Management menu)
Click Apply to save changes
This does not include the actual dimensions, but it does tell the printer to expect a custom paper size when you set up the print job using the printer software.
Example
Product Specifications for the HP ENVY Photo 7134 All-in-One Printer (Z3M48B)
Section Paper Handling
HP Document:
HP Printers - Creating Custom Paper Sizes
HP Printer Home Page - References and Resources – Learn about your Printer - Solve Problems
“Things that are your printer”
NOTE: Content depends on device type and Operating System
Categories: Alerts, Access to the Print and Scan Doctor (Windows), Warranty Check, HP Drivers / Software / Firmware Updates, How-to Videos, Bulletins/Notices, Lots of How-to Documents, Troubleshooting, Manuals > User Guides, Product Information (Specifications), more
When the website support page opens, Select (as available) a Category > Topic > Subtopic
HP ENVY Photo 7134 All-in-One Printer
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