Confirm the order and the size
Check the order and ask which size they want instead. Confirm that size is in stock before you promise anything.
State the exchange steps once
Tell them how to send the item back, what condition it must be in and when the new size will ship. Put it in a short numbered list.
Ship the new size early when you can
If your policy allows it, send the replacement once the return is scanned rather than when it arrives. It shortens the wait and costs little.
Offer a refund if the size is gone
If the size is out of stock, offer a refund or a different colour and say it plainly. Do not leave the customer waiting on a restock date you cannot promise.
Fix the cause on the product page
If one product keeps coming back for size, add a size chart, fit notes or customer fit comments. Fewer wrong sizes means fewer tickets.
Track exchange reasons
Tag size exchanges by product. A cluster on one item points to a sizing problem rather than a customer problem.