It really does sound like an endometrioma. With your past history, this is very likely. Ovarian cancer masses do not decrease in size. So, I think you can breathe a little easier that this is probably a benign cyst. Good luck with your surgery!
Actually...mine did decrease in size over the period of one year....and was indeed cancerous. So, while that may be the rule usually...it is not always the case.
It is good that a gyn/onc is doing the surgery. I hope it is benign!!