The video below was posted in this forum some weeks ago, it is an interview with a cardiologist (by coincidence happens to be my attending cardiologist) regarding exercise and physical activity while having an aneurysm. I had a 5.2 cm TAA and bicuspid valve, it was found while preping me for leg surgery after a MTB accident. I was very active, weights, bike, swim, and luckily the aneurysm did not disect or burst. Had surgery 3 months after leg surgery as the Drs determined it was large enough for my body size and due to family history of burst aneuryms. That was 5 years ago, exercising, not smoking is important, you want your body in the best shape possible for surgery, but there are limitations and any exercise that makes you hold your breath or grunt while exerting (lifting heavy weights, crunches, etc) have to be avoided (vasalva as @moonboy pointed out in his response) as it is known your BP spikes significantly momentarily while making that effort, and high BP is the worst enemy of an aneurysm. Yours is already in the critical size if it is at 5.1cm, you gave a pretty wide range.

Find the best Surgeon and Cardilogist you can, a cardiologist who is an expert in aneuryms, they exist, and can give you the best advice taking into account your own situation. I am glad I found Dr Prakash (Dr in the video below) I see him every year to monitor my BAV (it was not replaced during surgery as it was highly functional). He also orders a full aortic path MRI (3 sections) every 4-5 years to ensure no new aneurysms ara appearing.

5 years later I'm doing great exercising a lot still,

All the best and keep moving and taking care of yourself, but be sensible