Unfortunately eBay are not likely to do anything as it is the buyer's opinion.

No idea if you came across as rude and/or ignorant to the buyer, but clothes are notoriously difficult to get right.

Always best to put accurate measurements and if you get someone complaining they're not as described, get the item back for a refund unless you can show absolutely that it is buyer error or remorse.

Also, whilst that neg and probably the related low stars have not been a disaster as such, your seller ratings are slipping and combined, that will cause you a problem before too long.