I'm not sure what to tell you about the width of the mirror. But I have just done a bathroom that has sconces on either side of a mirrored medicine cabinet. The space between the cabinet and the wall is 9" on each side. An average sconce needs about 5" in width, leaving only 2" on each side. I think it would look crowded to put a sconce in a much smaller space. To me, 5.5 would be small.

I would get a 78" mirror and mount the sconces on the mirror. I would use 3 sconces. See picture linked below.

Here is a link that might be useful: sconce on mirror

I would do what pps7 says or something like what I did. (Cutting holes in mirror like that isn't cheap, so keep that in mind if it matters to you.) It'll otherwise be too tight on the sides and the mirror will, in my opinion, be oddly "off-centered" from the sink.

{{gwi:1387804}}

{{gwi:1499444}}