The below photo may help in your decision. The mirror is 24” by 40” and the sconce height is 15”.

@mark1993 Thank you! Where is this mirror from?

This recessed medicine cabinet mirror is 24” by 30” and the sconces are 16” in height.

The black mirror is from Restoration Hardware.

I think the simple linear sconces that are modern, look better if they are a bit longer. The sconces shown in the photo above, which are a more traditional style, and project out more from the wall, seem to look better in a shorter size.

I think you can go longer with this style of sconce:

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We're just finishing up with our bathroom remodel. The sconces were the hardest decision for me! To give you an idea of scale, our mirrors are 48 inches tall and the sconces we chose are approx 24.75". With your mirror, I think you could choose anything in the 15-20 inch range.

I like the long narrow sconce for modern designs. Love marc1993's picture

@KMK Thank you. By the way, my initials are KMK.