They wait more or less until the odds of a bad outcome from surgery are less than the odds of a bad outcome without surgery. This should be reassuring.

I am at 4.5 cm. My local cardiologist told me the change of the aorta failing resulting in death at this stage is extremely low, way below 1% (assuming I follow precautions like not lifting heavy stuff, straining, grunting etc). According to him surgery, depending on the hospital, has a 3% fatality rate some have said 5%. Also if it is kept stable and does not grow people can live with it long enough and finally go from something unrelated and never have to have surgery. The cardiovascular specialist/surgeon that follows my case (out of state) has told me the same.

Depending on your age and many variables including body surface area, height, weight, vessel patency, etc., contribute to risk of dissection. My husband’s medical center described a 4.3 cm aneurysm on a CT scan in 2007. He was not told about it but the comment was that this was normal for his height and weight. In the meantime, he continued with all sorts of very heavy lifting and other “risky” activities.
In 2021 a subsequent ECHO and CT angiogram with contrast show a 4.8 cm aneurysm. In May 2023, it had not changed.
He has been followed carefully, including ruling out vasculitis, and he has been advised to keep his BP low; not lift more than 30 lbs; etc..
We hope that this never gets to over 5.0 - or possibly 5.5. It is an individual decision as to when to operate. You want a high volume aortic aneurysm center.