How can you treat a pair of eyes that is small on the left but big on the right without any eyelid surgery?

You can't: There are many reasons for asymmetry in the appearance of the eyes. Virtually all would require a surgery to improve. Occasionally, Botox or a filler may improve the appearance though.

Need more info: I assume you are talking about asymmetry in the apertures (openings between the upper and lower lids.) this is usually due to a drooping eyelid on one side and a compensatory elevation of the lid on the other side. I am not aware of any treatment other than surgery.