So far as I know it’s based on size. See section 2.3 here: https://www.ahajournals.org/doi/10.1161/CIR.0000000000001106

It says: They found that, relative to a control aortic diameter of ≤3.4 cm, a diameter of 4 cm, a diameter of 4.0 cm to 4.4 cm conferred an 89-fold increased risk of dissection, and a diameter of ≥4.5 cm conferred a 6000-fold increased risk (Figure 5), albeit these are only relative risk estimates and do not inform absolute risk. It follows that the increase in risk at 4.0 cm to 4.4 cm justifies defining an aorta of this size “dilated,” and the abrupt increase in risk at a diameter of ≥4.5 cm justifies defining an aorta of this size as an “aneurysm.”

I’m not a medical professional though so there may be conventions in the community that don’t follow this.

I have a dilation of 4.3cm. I think the absolute risk of problems at this size is pretty low. I try not to let the relative risk scare me unnecessarily. At 3.9 I’d guess your risk is pretty low. I just try to monitor my blood pressure regularly, keep it at low end of normal, and avoid straining/heavy exercise.