Has he opened a return case for ' item does not fit' ? If not, then, with a No Returns on your listings you can refuse the return for those reasons.
If they are returning for a case of item not as describe you will have to refund them but only when the item is back with. Cases are all automated , so no point challenging it at that point, but if refunded you can appeal the case for a refund. I will admit it's not easy, but if you have the proof that all measurements were in the listing , then it's abusing the MBG for a refund by claiming it's too small.
This explains how to appeal:
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
@billybags734