Save all of your eBay emails! Make sure when you correspond with her that you do it through the "contact buyer" on eBay, not through private email. If you can prove that she bought the same thing a few days later, especially if she leaves positive feedback that she is happy with her purchase from that buyer, then say "no way!" and let Paypal handle it. I agree: it sounds like buyer's remorse, and to think she would return it for such a reason as the size!?! Not that it is damaged or defective, but that she doesn't like the size. Stick by your word on this one. Make sure you state your side to Paypal very clearly, without losing control or getting defensive.
Heck, she can always sell it herself on eBay if she isn't happy!
(in general though, you ALWAYS want to make sure to list the measurements on a handbag w x h x d and strap length, if applicable.)
Good luck~ let us know what happens.