They should probably image using mri as its less user depent and has less variation.
It depends which portion of the aorta is being measured; for example, aortic root at level of sinuses of Valsalva, aortic root at level of sinotubular junction, or ascending aorta of level of pulmonary arteries. In general, an ascending thoracic aorta of 3.8 cm is top normal/ectatic, 4-5 cm is dilated, and greater than 5 cm is aneurysmal, although the exact cutoffs vary depending on the source used. Overall, this does not sound serious and can probably just be monitored with routine surveillance.
If further evaluation desired, CTA or MRA chest would be able to provide more accurate measurements.