One of my eye has started to appear smaller than the other all of a sudden. I'm not sure if the issue is the small or big eye

This also means that you need to be assessed for hyperthyroidism.  The upper eyelid retraction and prominent eyes suggest new onset of Graves disease.  There are individual who show no signs of hyperthyroidism and simply present with eye changes.  That is called euthyroid Graves disease.  Systemic signs of Graves disease are those of overactive thyroid-weight loss, nervousness, heat intolerance, increased bowel movements, easily fatigue.  If these sound familiar, see your primary care physician and get a referral to an oculoplastic surgeon.

You should consider seeing an oculoplastic surgeon to rule out lid retraction and lid lag associated with thyroid eye disease.Be sure to bring in old photos to compare date of onset.