This is the last thing that any eBay seller ever wants to have to deal with, the unhappy customer, but sadly it comes with the territory of selling online. Even if you documented everything to a T, and shipped in a timely manner, you will still end up with the occasional customer who probably would not have been happy regardless of what you did.
These customers commonly fall into 2 categories, the ones who contact you first, and the ones who air their distaste on your feedback without ever thinking to contact you first. If you haven't already guessed, us sellers prefer our unhappy customers to fall into the first, and not the second category, as it gives us a chance to right a wrong before feedback is left.
On the rare occasion I have had an unhappy customer, I went out of my way to make it right. I have accepted a return when my listing specified I did not accept returns, etc and most times it resulted in a happy customer and positive feedback for me. I have also had the rare occasion where I tried to work things out, but the customer wanted more, ie full refund due to not as described, when policy stated refund minus shipping charge. These occasions commonly resulted in negative feedback, which sadly I was unable to reciprocate because eBay removed the sellers ability to leave negative feedback for a difficult customer, even though they can still leave it for us.
And for those customers that fall into the latter category of negative feedback with no communication? I have found it best to reply to the negative feedback with your knowledge of the situation. If they claim sizing problems, reply accordingly based on what was mentioned in your listing. It is instances like this when it pays to document as much as you can in the listing. If you provided detailed sizing measurements and the customer believes the size was wrong, it is their fault for not reading the listing in further detail.