Heartbeat in stomach. Always there, but how big a heartbeat there before you worry about abdominal aneurysm?

You are worried. Since you are worried now, you should see your doctor. If there is reason for concern, abdominal ultrasound is an easy and reliable way to identify abdominal aneurysms.
Depends. Very thin individuals may be able to palpate their own abdominal aorta, which could be of normal size. However, most individuals notice an abdominal pulsation when the aorta is larger than normal or aneurysmal. Please have this evaluated, an ultrasound or ct scan can provide definitive information.