I just had a look at my Excel version, but the only place I can select a paper size is in the Print dialog, and this is linked to the printer driver as PDR1 already mentioned. The dialogs for Page Setup and Set Print Area and such are from Excel, the Print dialog is mostly dependant on the printer driver.
One thing that may have happened, apart from your switch to a newer Excel version, is a Windows update. Did you perhaps also change the Windows environment to a newer version? Even though the printer may be the same, a change to another Windows version may have driven a change to another printer driver in the background.
Can you select A5 paper when you print from a native Windows application? You could try opening your browser (use Internet Explorer or Edge) and printing a page from that.