If they open a dispute for item not as described paypal will instruct them to return the item via trackable post at their expense to get the refund of the total they paid to you.

You can ask the buyer to return the item and IF you were wrong you will refund the cost of postage both ways but if the measurements are correct you will return the item once postage is paid to return the item or refund the cost of the item only.

If the measurements are correct send them a photo with the tape measure on the item where they claim it is wrong.

If they do open the dispute and the item is correctly advertised you can call paypal and tell them you can send photos to verify the item is as described and not to refund the buyer.