I have a 4cm ascending aortic aneurysm at what size does it need to be repairde?

Depends: Typically an ascending aortic aneurysm is repaired after it becomes between 5.0 to 5.5 cm in size, depending on other factors such as age, how quickly it is expanding, and the presence of other conditions such as marfan's disease.

Ascending aorta: 4cm for the ascending aorta is upper limits of normal. If the ascending aorta becomes larger than 5 cm or if you develop aortic regurgitation you may need surgery.

5-5.5cm: It depends on your specific circumstances, but that's usually the answer. Here are the official societal guidelines. See page e320. http://circ.ahajournals.org/content/121/13/e266.full.pdf+html.

At around 5cm: It needs to be followed up with ultrasound every 6 months. When it grows too fast or has reached 5cm then it needs to be considered for repair