An unknown issue causes AirPrint to report that a certain paper size is not supported by the printer, where it should be supported. It is usually as if only one paper size (such as US Letter) is available to print on, so this probably happens as a result of a problem in the paper size sensing system.
It only affects some users, some of the time, in some of the apps.
- Try emptying/reloading your paper tray
- Try turning the printer off and on
- Try restarting the iPhone/iPad.
- If none of the above solves this problem, try deleting Print to Size and re-installing it from the App Store. Some people have reported that doing so has made the issue go away for them.