I would just send a polite reply that as she chose to pay through Paypal and have it delivered you would prefer her to go through official Paypal channels if she wants to open a dispute for item not as described. If she goes as far as opening a dispute be very specific in your response regarding it being the size specified on the label and that it was brand new so there would be no damage on receipt. There is a 50/50 chance you will win if your response is good enough.

If Paypal find in her favour then she will have to return by post using a fully tracked service which will cost her probably more than it cost you to send it, if she isn't up for paying that cost she will not get a refund.

It is likely she will give you a neg anyway unless you pay her back in cash but don't allow that to sway you, better a red dot on your computer screen than risking being ripped off.