According to the search results, the size of a kidney cyst is not the only factor that determines whether it is dangerous or not. Simple kidney cysts are usually small round sacs that have a thin wall and are filled with a watery fluid. They can range in size from a small pea to as large as a grapefruit, and they can also grow over time. Simple renal cysts are considered large if they start causing problems, such as pain, urinary blockage resulting in kidney swelling, or other issues. Doctors may consider a diameter greater than 3 cm worthy of a follow-up, but they will typically do imaging tests to better look at kidney cysts because, in some cases, there may be something else going on. It is important to note that simple kidney cysts are almost always harmless, but some cysts do have thickened walls, may look irregular on X-rays, and could be associated with kidney cancers. Therefore, it is crucial to consult a specialist if you experience any symptoms or have concerns about kidney cysts.