I have a pituitary adenoma (16mm).My prolactin is 115, 3 (normal values 2, 8-29, 2ng/ml).I'm on thyroid medication. Probably have PCOS. Surgery or drugs?

Prolactinoma. You have a prolactinoma until proven otherwise. The best initial treatment is a medication that inhibits prolactin secretion from the tumor and acts to shrink it. The most popular one at this time is cabergoline that is taken twice per week. You may then watch the prolactin levels with periodic blood testing and watch the tumor with an MRI once per year or so. Rarely do you need surgery.
Probably surgery. You probably need to see an endocrinologist or a neurosurgeon. If this gets too large it can affect your vision. They may recommend a trial of medication, but ultimately they will do a gamma knife.
Likely need surgery. That is a sizeable adenoma. You likely need surgical treatment.
Depends on the size. Prolactin level is irrelevant. You definitely do not need surgery if the adenoma is small, but if it is large and causes visual problems, then it needs to be taken out.