Can’t Change Printing in Pages on iPad

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Use AirPrint to print from your iPhone or iPad - Apple Support

When using AirPrint, the available print options - including page size - are advertised by your printer. When using the print dialogue on your iPad, you can only select from the advertised options.

Some printers can detect the media that is loaded in one or more of its paper trays, others may have settings that must be set from the printers controls, by accessing a built-in web server using a web browser, or using a companion App supplied by the printer manufacturer. By example, HP’s AirPrint printers can be managed using the HP Smart App.

I find that in my case (an Epson printer), I must correctly set up the proper paper size on the printer itself (in its settings). That is what comes through to Apple's AirPrint device.