We have a 19" (24" with trim) recessed medicine cabinet/mirror over a 42" vanity in a master bath used by two adults and find it works well--it isn't full, nor are the drawers in the vanity. We do have a full-length mirror just outside the bathroom. Whether it's enough mirror and/or storage space for you will depend on who uses the bathroom. Our hall bath has the same size cabinet and it's only half full, but that bath is used by two kids who really only keep toothpaste, etc. in there. I would not have wanted anything wider with sconces, and we wanted sconces vs. an overhead light. Ours are not that wide, though--around 5". Only an inch of space does feel very close to the wall.

Another option is to see if you can find a similar fixture at a slightly smaller scale. Rejuvenation sells one (Knowles) that is very similar that is only 5" (6" with shade), for instance. Other companies might also make similar slightly smaller fixtures. Those extra inches might be valuable for you.

Thanks artemis78! Yes, we decided to go with a 20" wide cabinet. I love the Robern Murray Arch - but the mirror front is bisected at top. We'll give up on the integrated electrical and go with the Kohler Verdera capsule. Our GC recommended something longer than the single globe sconce - like a two globe sconce from RH. However, I think two globes is too much - one too many round item (times two since there are two of these on the wall!). Visually, we need a little length - but I'm not finding anything linear that doesn't remind me of a saber. The RH double globe sconce comes in a single design that's as long as the two-globe design.