Hello Paul and welcome to HealingWell. Great question and I wish I had some words of wisdom but this is a bit over my head ( no pun intended). I will make a couple of comments but please remember I am not a physician.
There are many factors that go into deciding when to operate and what approach. The location of the aneurysm, your health history, and other medical factors need to be consider in a case by case manner before a Dr. suggests surgery. In a patient that is healthy, non-stressed and has no other heart problems the Dr. may suggest treating the patient conservatively until they get to 5 to 5.5 cm. At 5.3 cm you are in the group that frequently is treated now, not a wait and see position.
Be your own best advocate and seek out another opinion re surgery from a well known surgeon that has performed this surgery successfully.
Thinking of you and keeping you in my prayers for a successful out come no matter which treatment you choose.
and Heart/Cardiovascular Disease.
"She Stood in the Storm & When the Wind Did Not Blow Her Away, She Adjusted Her Sails."