How do I change the page size for printing from iPad?

See:

• Use AirPrint to print from your iPhone, iPad, or iPod touch - Apple Support
• Connect an AirPrint printer to a Wi-Fi network - Apple Support

Apple AirPrint is a driverless connection protocol - that requires a compatible Printer to be wirelessly connected to your WiFi network. A high proportion of modern printers intended for Home, SoHo and Business environments now support AirPrint. Check your printer model against this list:

About AirPrint - Apple Support

Assuming that your printer supports AirPrint - and has been correctly set-up - your iPad will display the available print and page options within the print dialogue. Key here is that you can choose from print/page options supported by the printer over AirPrint.

For example…:

If instead you are seeking methods by which you can influence the page layout prior to printing the page/document, this isn’t a facility supported by AirPrint. Instead you have to arrange the page prior within the App from which you intend to print the document. While some Apps have the ability to influence the page layout, many don’t.

For Apps that don’t have this facility, you must instead use a dedicated third-party printing App that provides page-layout options. Sadly no longer available for purchase, Readdle Printer Pro was a good example of printing Apps that allowed some flexibility of page layout, including the ability to scale the page (e.g., change margins etc.). There are doubtless other Apps available, but you’ll need to find something suitable for your needs in the Apple App Store; Googling might help you to find some possible options.

I hope this insight provides some useful information, or provides a better understanding of how you might achieve your goal.