Problem selecting A5 size paper.

About two years ago, I similarly experienced difficulties with AirPrint and an older inkjet printer and print server that had ceased receiving support by its manufacturer. Direct printing over USB from a Windows computer has no issue, but AirPrint would not allow selection of print options or paper size.

As the printer ink cartridges were themselves becoming increasingly difficult to obtain - even as remanufactured units - the opportunity was taken to replace the printer with a modern/current printer. With this, all issues with AirPrint were resolved.

While not necessarily suggesting that you should replace your printer, consider that software updates (including print protocols) do evolve over time. Eventually, host software may not be able to successfully communicate with older devices.

Start here:

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.