My left eye is bigger than my right and when I smile I don't think it looks good. Does lack of sleep cause my eye to be smaller?

A picture would be helpful, but I suspect the problem you are referring to has to do with a difference in the position of your lower eyelids. Lower eyelid asymmetry when smiling is a common problem and not likely related to how you sleep. If you like the lower side better, the higher side can be treated with low dose botox injections to help make your face more symmetric.

It’s difficult to say the exact cause without a photo.
Differences in the appearance of the can eye can be related to position of the
eyeball within the socket or differences of the eyelid. Some people have can
have abnormally drooping eyelids. This is called ptosis.

Regardless of the cause, consultation with your local surgeon will help
determine if there is a treatment to help you.

Lack of sleep will not permanently change the shape of your eyes.  Your eyes may be slightly more swollen in the morning due to lying down all night but this resolves during the day.  The shape of your eyes is due to bone structure underneath and all patients have some degree of asymmetry.  See a board certified plastic surgeon if this concerns you greatly.

Everyone has eye asymmetry. Lack of sleep can affect the eyes in other ways, but it won't make one eye smaller than the other. Sometimes the eyelid can droop, however, and this can give the look of a smaller eye.Consult with board-certified plastic surgeon or oculoplastic surgeon for an evaluation of the asymmetry and to see if anything is correctable.

It would be great to see a photograph because there are several potential causes for asymmetries.Not sleeping enough may cause some occasional swelling but the effects are not permanent. Be sure to have your eyes evaluated by a board certified surgeon with expertise in eyelid procedures.

Lack sleep usually does not result in shape differences in the eyes. The underlying cause for asymmetries in the face are usually due to skeletal asymmetries. A photo would be helpful. Thank you for your question. Best wishes.