These are the 2 different sizes of wall sconce

First, If you choose to place your scones above the mirror, the width of your fixture should be at least 1/3 the width of the vanity mirror, but should never exceed its total width. In bathrooms with two sinks, another great option is to mount two separate fixtures, one above each sink.

Secondly, for an average size bathroom, buy bathroom sconces that are about 18 inches tall or a little less. For smaller spaces, try smaller sconces of 12 inches or less. In a larger bathroom, you could consider sconces that are 19 inches tall or even taller!

A good rule of thumb is to hang sconces anywhere from 60” to 70”. Wall mount the sconces on both sides of the mirror. Ideally, they should be 36″ to 40″ apart to ensure even lighting on the face

In conclusion, its about which gives you enough lighting and still fits your interest. Considering the width of your 24" H by 18" mirror, it is still okay to use the 20x6 scones.