The paper sizes available in Word are taken from the available sizes in the printer driver.
If you have had A5 available in the past, check that the correct printer is selected and is being used.
My default printer is an Epson capable of A3+ sizes, if I select another printer, a HP Laserjet, A3 or A3+ is not shown as that printer can’t physically print to that size.
If I choose say Send to Onenote 2010 or 2016 as the printer, A5 isn’t shown as it’s not available.

Sometimes the wrong printer is selected, often accidentally, so have a look to see what is being used at the time of an A5 no show.