By lower aorta, you probably mean the abdominal aorta. Normal diameter is 19 - 25 mm. After the age of 50 years, the diameter is usually around 2mm or more. So, 5.3 cm is very large. Surgery is recommended for diameters more than 5.5 cm and when risk of surgery is less than risk of rupture. I suggest you follow-up with your vascular specialist.