"Doesn't fit" is classed as change of mind. You needed "item not as described". You will have to try to contact someone at customer services and ask them to change the reason. eBay have made that harder now but keep trying.
Click on the help link at the top of any page. Select a topic then scroll down to Contact Us. Get to automated assistant and put in the question box Can I speak to an agent? Keep repeating until they pass you to a real person.