I found out I have an ovarian cyst sizing at 9cm x 4cm. Is this a big size or no?

Yes! You need to have a discussion with your doctor - or a doctor - tomorrow about managing this.
Yes, it is big. This is likely considered a large ovarian cyst by most standards. Size is not the only criteria we evaluate. Presence of septations or nodules in a cyst are important additional information that can be seen on ultrasound. Your doctor may recommend surgery depending on your symptoms, appearances of the cyst on us and whether it grows over time.