Custom paper size air printing on IPad from 3rd party software

With AirPrint, what you see is what you get. There are no native means by which to add customs print options or paper sizes.

You may find third-party printing Apps in the Apple App Store that might be helpful in expanding printing options. One that comes readily to mind in Readdle Printer Pro.

Edit: Printer Pro appears to have been withdrawn - and is no longer available for purchase or download.

Support for advanced printing features - and differing paper sizes - is largely dependent upon your installed version of iOS/iPadOS. Most third-party Apps will completely reliant upon AirPrint services.

Natively support printing relies upon Apple AirPrint. AirPrint is designed to provide basic printing services for printers that themselves support AirPrint. In iPadOS 15.x, AirPrint capabilities have been enhanced.

Your iPad will only expose print-options that are available for your installed version of iOS/iPadOS and your printer.