It depends on what the return policy was. Go back to the ad and look. A standard section in ebay ads requires the seller to fill out what she/he does regarding returns. In the event your situation does not meet the sellers requirements for returns, try contacting them and expressing your concern/issue with the bag. Most well reviewed sellers will work with you in an attempt to make you satisfied. In the end, unless the product was highly miss represented by the seller, or damaged, you really can not file a complaint to ebay for money back. Your best bet is communicating with the seller
When a seller doesn't respond you don't buy it. It sounds like there was nothing misleading in the listing. I doubt you have a case.
If the size wasn't listed and the seller doesn't take returns, you are SOL.