Why does one eye look smaller than the other and why is it not even with the other eye? (photos)

The difference is very subtle, but you do appear to have slight descent of the brow on the right.  If this occurred overnight as you state, it may be a very mild form of Bell's palsy.  The fact that it is still present at 2 mos makes that somewhat less likely, but still possible.  I would wait until around 6 mos before considering any surgical correction, but a browpexy would likely correct your asymmetry.

Everyone has subtle difference in the shape of their eyes.  As an example, one eye will have a more round appearance and the opposite will be more oval in shape for the typical Caucasian patient.  Your photos do not suggest a pathologic degree of difference.

There is not an explanation for everything.  Since you CT did not show any on going problem nothing needs to be done. Your ophthalmologist is giving you correct advice.