Eyeliner is your best bet when trying to create symmetry with your eyes. Adding more eyeliner above or below one eye can create the illusion of shape. Simply draw the line to better mirror the eye that is bigger.
Adding mascara and eye shadow will help to make the eye liner look more natural and less like a thick black line. Using a darker eye shadow, dust it along the crease of one eye lid. Do the same to your other eye, but feel free to put a little below or above the crease to make it seem bigger or smaller, respectively.
Don't go overboard with the eyeliner or eye shadow. Subtlety is what will create the illusion that your eyes are symmetrical. There are many makeup tutorials online that can show you how to properly apply makeup.
Most importantly, do not forget the mascara. If you have naturally thick or long lashes the mascara will help blend the liner with your eye to make it look more like your natural eye line. If you have smaller or thinner lashes you may want to consider false eye lashes. Keeping your lashes symmetrical will help to keep the entire look symmetrical.