Which side is abnormal?!

Failure to dilate, failure to contract (usually exogenous anticholinergic, such as Hyoscine patch gone AWOL)?

Is eye lid sagging?  Is skin dry?  Reflexes?

Horners is small pupil with ptosis and dry skin – apical tumour (Pancoast) or lesion in brainstem/neck.

Dilated pupil can be part of third nerve palsy but you would usually also see ptosis and loss of adduction. Adie pupil is sudden onset mydriasis (dilated) with loss of reflexes (usually just ankle jerks) on the same side, can spread to other side.  Usually post viral. Can be excessive sweating. Loss of reflexes can be permanent.

Physiological anisocoria = size difference does not vary according to light/dark.