I use Office 365 and have the ability to select A5 on my printer at home too. How many printers have you actually got installed (see Settings>Devices>printers & scanners)? The reason I ask is that I found this on a Microsoft forum
I guess another thing to check is which driver are you actually using, the OEM driver or Microsoft's own generic driver, better to use the OEM driver if you have it IMHO.