Why Are my Eyes Different Sizes?

Looking at your photo, there appear to be a couple of differences between the two sides.  First of all, the left brow is elevated compared to the right.  Often this is simply a physiologic/normal difference, but it can suggest the presence of an underlying issue such as a droopy or ptotic left upper lid.  In this case you use the brow to elevate the lid to a more normal position.  There is a slight retraction of the left upper lid that may be related to the brow, or might be associated with other issues such as thyroid abnormalities.  It is also possible that the left eye is more prominent, either congenitally or due to an orbital process.  Certainly there are other possibilities as well.

I suggest you see an oculofacial plastic surgeon to further identify the underlying process and how best to treat it.  Try the ASOPRS website to find a physician.

Asymmetry between the two eyes can have various causes. Some of these causes include:
difference in upper eyelid positiondroopy eyelid (eyelid ptosis), eyelid retraction, etc. difference in lower eyelid positionreverse eyelid ptosis, eyelid retraction, etc. difference in eyeball positionsunken eye (enophthalmos, hypoglobus), bulging eye (proptosis, exophthalmos), etc. orbital asymmetryfacial asymmetryetc. 
The treatment depends on what is causing your issues. You should see an Oculoplastic surgeon and Ophthalmologist for a complete evaluation. 

Everyone has facial asymmetry.  It can be more noticeable as we age.  Typically one eye is slightly larger than the other eye and that is completely normal.  A blepharoplasty can help improve overall symmetry if done by the right technique.  Be sure to seek a consultation with a board certified plastic surgeon or oculoplastic surgeon with experience and expertise in eyelid surgery.

The shape and size of the eye is affected by many different structures around the eye, including the eyelids, brows, bony orbit, and cheeks. In the included photo, there appears to be some brow asymmetry with the right brow lower than the left, which subsequently makes the left upper lid appear more open. This is something that could be corrected with Botox to the forehead to even out the brows, which might also make the eyes appear more symmetric by lowering the left brow and consequently showing less of the left upper lid.
Be sure to see an expert who specializes in facial aesthetic procedures.
All the best,

Many different situations can lead to uneven eyelids. Some of these can be treated non-surgically and conservatively, even using some very advanced injection techniques with filler or very precise placement of Botox. Some situations truly do require surgery to resolve. 
It is important that you seek a consultation with an eyelid expert to delineate the exact cause and treatment options.
To ensure you are receiving the highest level of care, seek out a modernly trained, new-school dermatologic surgeon, oculoplastic surgeon, facial plastic surgeon or plastic surgeon who is board certified and fellowship trained in one of these "core four" cosmetic specialties. Membership in organizations like the American Academy of Facial Plastic and Reconstructive Surgery help to identify a highly trained surgeon.
Cameron Chesnut
#realself500 Physician

This is much more common than one would think. If you look closely your left brow is higher than your right. I think that is the main difference. Again this is common if you look closely at many faces. A unilateral brow lift can help with this.  It is possible you have a bony asymmetry that is underlying. Sometimes it can be a hypoplastic maxillary sinus. That just means the cavity in the middle of your face is smaller on one side than the other. It has no importance unless you are to under go a sinus surgery it should be noted. Hope this helps.Best of luck,DrC