I don't know when you are saying ''eye doctor'' you mean neurologist but it is a neurologist that you must see to rule out any medical condition.
Uneven pupil size (anisocoria) might also be related to stress and anxiety once you rule out any neurological issue. One reason might be that perceptual information from the eyes, ears, and rest of the body is sent to the right hemisphere therefore perceived stress can increase the left pupil size. I know i said right hemisphere and we are talking about the left eye but the brain's hemisphere work in a way that the left part of the brain control the right part of your body and the right part of the brain controls the left part of the body.
I know emotions are processed bilaterally but it can be a whole different story when talking about stress and one part of your brain might be under more perceived stress resulting in anisocoria.
Saying that your anisocoria is med related might right too and it have been observed in people taking anti-depressants but the underlying cause is not understood but here above you can have a clue about one underlying cause. Maybe try switching Effexor to Cymbalta ( same class of drug ) of Lexapro ( different class of drug ) to see if it goes away.
Anyway please consider that I'm not a doctor and those are issues that must be discussed with your doctor.