13 inches???? that's huge. Are you sure it isn't 13cm? Yes, I would think anything of that size would need to come out.
Such a huge ovarian mass can be an ovarian cyst or a benign ovarian growth. It can be malignancy even. Amongst the common benign ovarian growths, mucinous cystadenoma often causes such huge ovarian mass filled with mucinous material and rupture may cause pseudomyxoma peritonei. Fine-needle aspiration and cytology may be used to confirm the impression that a cyst is benign. A CA 125 may also be done to check whether there is any malignancy.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.