The size of an ovarian cyst can vary depending on the type of cyst, and most ovarian cysts are small and benign, measuring less than 5 centimeters in diameter. However, a cyst larger than 5 centimeters is considered big and may be harmful. Doctors may consider an ovarian cyst large if it measures 10 cm (about 4 inches) or greater, but this can also depend on the type of cyst. A cyst larger than 5 cm is considered complex, which can potentially become cancerous. Ovarian cysts larger than 5cm will be considered by the doctor for surgery, especially if they cause severe symptoms or continue to grow. An ovarian cyst larger than 4 cm in diameter has a greater chance for torsion than those that are smaller. In conclusion, the size of an ovarian cyst that is considered dangerous depends on the type of cyst, but generally, any cyst larger than 5 cm can be concerning and may require medical attention, including surgery.