You have a No Returns on your items so for a case of item does not fit, you can refuse to accept that case.
However, if the buyer finds ' fault' with the item and opens a case of item not as described you will have to accept this pay for the return tracked postage and yes, you do have to refund everything the buyer initially paid.
To avoid this in the future when selling clothing it's always best to include all relevant measurements, as a Size 18 in one store / Brand can be very different in size to another store's item. Always add waist, chest length measurements whichever are relevant , and you can avoid many returns.
@missmonkey2014