You will be forced to give it back if they sue, although you could, potentially, be able to counter-sue and ask for the amount you spent re-sizing it.
The reason is that there was no contract formed for the ring since they didn't intend to give it to you.
It may be that they just forget to follow up with you on this. I had a friend who had a similar thing happen with a painting although he had actually considered buying the painting while on the cruise which is how, we assume, that they got his name and address mixed up and sent the painting without it being paid for. With the painting they eventually made arrangements for one of the delivery companies (I think it was UPS) to pick it up.
They will be entitled to get it back but you can take the position that they need to send you something that allows you to ship it without cost to yourself. You may even want to get a local lawyer to write them a letter telling them this and also asking for the fee for resizing. The lawyer isn't going to charge much, if anything, just to write a letter for you.
If your question has been answered then I'd offer my best wishes to you and ask that you please not forget to leave a Positive Rating so I receive credit for my work. Of course, please feel free to ask any follow up questions in this thread. I want to be sure that all of your questions are answered.