If the size was misdescribed, and a case for item not as described is opened you will have to accept and fully refund on the return of your item. The case will explain you need to send a paid paid tracked postage label for their return.
If they are just saying the correct size was ordered but they do not fit, if you have a No Returns then you do not have to accept a return for that reason.
Doubt the buyer would be happy for any partial refund offer to keep a pair of shoes which do not fit.
@deoha14