Ultrasound of pelvis showed 6 cm cyst. do i need surgery?

Depends: Your ob/gyn will be the one to decide. It is very hard to tell if you need surgery or not based only on the size of your cyst.

Maybe: Decisions for surgery are based on discussions between the patient and clinicians, which in this case should be you, your ob/gyn, and possibly the radiologist.

Yes if persists: Depends on the location & type of cyst. A simple cyst (thin-walled, only fluid) can be followed if on the ovary, if it resolves spon, you're done. If it's complex (thick-walled, septated, nodules present, vascular, etc) it needs surgical excision to r/o neoplasm. >6 cm can also torse & cause severe pain so we usually recommend laparoscopy to evaluate & excise the cyst, surgical staging if ca risks.