Hi, it is scary, I know. Women get "functioning" cysts each month during their reproductive lives. They are supposed to go away. If they don't, doctors either watch them or, if the cysts grow, take them out. Since your cyst seems to have shrunk a bit your doctor most likely wants to see what it will do after you have another period.
Cysts that big can cause pain and other symptoms. I'd suggest keeping track of the pain or anything else you notice that is out of the ordinary for you.
Try to relax. I know it's hard to do. Take care.
Good luck and please update us when you hear anything else! I am going through the same thing right now and would like to know what other doctors are saying about complex cysts.