My understanding is that the cutoff for surgery does vary. I'm not sure about giant cell (arteritis? arthritis?). There are some genetic conditions that move the threshold for surgery lower. The link @rlhix posted discusses this. Those are guidelines, which means they may not include the latest research (not that there is any).

Even without the giant cell arteritis as a probable cause, the decision on whether to have surgery at 5.0 or 5.5 is being studied. If you spend some time googling, you can probably find a page that describes the clinical trial. For multi-disciplinary teams and a high-volume center, there is some evidence I think that the risk of surgery at 5.0 may be less that than the risk of dissection. The key point being multi-disciplinary team at a high-volume center. That said, I have no medical background. That's just my take from the reading I have done after being diagnosed with a 4.5 cm aneurysm. Coincidentally, I had PMR at the time of the diagnosis and wondered if it could be related. I think not and I never had giant cell arteritis.

BTW, did they scan your abdominal aorta?