I just had a look at some of your listings for different hammocks and they ALL had measurements, so I would be fighting this to the death. If the numpty buyer couldn't get a tape measure out BEFORE jumping on the BIN button, that's HER problem, not yours.

Call eBay first thing tomorrow and start the ball rolling. I have read somewhere on here where the decision was reversed and the seller was refunded the full amount that eBay had given the buyer, so it can be done.