A family of Microsoft word processing software products for creating web, email, and print documents.
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In the mean time, on the Layout tab, open the Page Setup dialog, choose the Paper tab, set the Paper size dropdown to Custom size, then enter the dimensions for A5 or A3 paper to create your preferred page size.
You don't necessarily need to own a printer to be able to set a chosen printer in Word. You can install a printer driver that includes A5 and A3 sizes. Then select that as the printer in Word.