What size wiper does a 1998 Chevy silverado use?

Assuming you mean a C1500 Silverado, it takes 18" wiper blades on both sides front. See related links below. Nowadays attachment is just as important as length, so always lookup exact part numbers by vehicle make, model and year to ensure not only the correct length, but also the correct attachment. Not all wiper blades fit all 22+ wiper arm attachments.