I have a thin walled cyst(anechoic) in my left ovary whose size is 65mm*56mm. 4 months back the size of cyst was 61mmx57mm. is it harmful?

Stable cyst: Based on what information you're giving it sounds like you have a stable sized ovarian cystic tumor. Probably a benign serous cystadenoma. The main issue is the size of it and the chance or twisting (torsion), which can cause severe pain and loss of the entire ovary on that side. Since its not going away you probably should consider surgery.