Noooo! Depending on how you portion your shipping, you do NOT have to refund in full if the buyer stated that it was a fit issue in the return comments section or through messages. I just processed a return today for this very situation. I refunded purchase price only, no original shipping (and should be seeing a credit for return shipping--if not, I will be calling in). My returns specify that the buyer pays return shipping. The original shipping is a separate charge and not part of the purchase price. It would not be refunded in a buyer's remorse situation. This is why I rarely offer free shipping on clothing items because there is a high rate of returns in this category. If you offered free shipping on your listing though, you are out of luck.
My buyer escalated to a case to get her original shipping refunded. It was found in my favor with no impact to me, and although eBay did issue a courtesy shipping refund to her, it did not come out of my funds. I'm pretty sure eBay keeps track of those courtesy refunds, so it will eventually catch up with her.
So, when you get the item back and go to process the return, there is a choice to report a problem to eBay (it's just under the refund payment option). Select this and one of the choices is that the buyer wrongly claimed item doesn't match description or pictures. Select this and make sure to leave very clear details that this is a fit issue and reference the message/ comments from your buyer to prove this. At this point, you are given the option to refund a different amount. In the past, I've called in for this situation, but the new option worked great for me and saved me time on the phone. You can also report the buyer separately under the Report a buyer link in Seller Hub.
Most returns go smoothly with the buyer accepting responsibility for fit issues, but occasionally I get someone who acts very entitled. You may know the ones... I have always worn a size 6 and these jeans don't fit like my others so it must be that the item is not as described, even though the jeans are clearly marked a size 6 and accurate measurements have been generously provided. Over the years, I have won many wrongly opened SNADs over fit, so don't let others tell you that you can't win. If more sellers stood up to these buyers who abuse returns, there would be fewer of these false claims. Every time you give in, you create more of a monster.
