Do I have ptosis? One eye became smaller than the other one (photo)

You either have left upper eyelid ptosis or sunken left eye or both. See an oculoplastic specialist for evaluation. 

Eyelid ptosis is extremely common and often is hard to see since most patient compensate especially on the dominant eye side by using the forehead muscle to lift the eyebrow and to help open the eyelid. This can be eliminated through very small outpatient procedure. You definitely have ptosis on the left side but it could be on both sides and you could be compensating by raising your right eyebrow. Without seeing the entire forehead this is hard to conclude.

Hi Selena. Greetings from the UK. Looking at your pictures, yes you do have a minor degree of ptosis affecting YOUR LEFT upper eyelid. Do you wear contact lenses as this condition is more common with contact lens wear where the edge of the contact lens causes stretching of the levator aponeurosis tendon. It'll often be worse in the morning and be more apparent when you're tired. I'd recommend that you see an oculoplastic surgeon who'll be able to examine your eyelids and may recommend ptosis correction surgery.