Robern is good. We have 90" of space above our two sinks, which are on an outside wall. What we did was to put high density insulation behind the cabinets, recess them partway, then put molding around the cabinets to cover the exposed sides. It actually looks much better than either a recessed cabinet or a surface mount. If you have a vanity, as opposed to wall hung or pedestal sinks, see if you can get trim to match your vanity. I have a magnifying mirror between the two sinks. My DH uses it for shaving; I use it for makeup.

I clean my medicine cabinet mirror with Windex when it needs it, not even close to once a week. :) The wear isn't on the surface, just along the edge where I touch it to open it. I don't manhandle things, so this is clearly gradual erosion of the edge. I had wanted a simple beveled edge, but thinking a very narrow frame might be in order.

Can you mock up what the mirror and vanity will look like - use some painters tape to indicate where in the room the vanity will end and then outline the various choices of mirror/mirror cabinets. You might be surprised at how some work better than others. With the Kohler Verdera 101cm (40") by 76cm (30") medicine cabinet, it doesn't matter if it's 7" shorter than the sink - what matters is if you are using the sink to wash up, and you look up - are you able to see yourself properly in the mirror? IOW, is it functional.

They are typically '16"' because that's the size of your framing. It's actually 15 1/4" or something to fit between the studs. It would be fine next to your vanity. I have 4 of them in my current house, and had planned for one in the new house, but haven't gotten to it yet. But, that's assuming your builder hasn't done anything funky with the framing and plumbing. In my case, there's a 25" wall next the vanity, and then a door through the wall right in front of the vanity. Rather than having a 16" cavity, and then a 6" cavity and then the support studs for the door header, the framer put a stud right in the middle of the wall. No medicine cabinet would have fit. I noticed it and had the middle stud moved over. And then the plumber came in, and ran pipes EVERYWHERE. Seriously, I think he added extra pipes just to fill my wall cavities. Every single wall in my second floor has plumbing in it. And in some cases, he's run 3 vent pipes in cavities next to each other, and then tied them together at the top of the wall and run 1 out the roofs. We got lucky (or maybe the GC smacked him) and there isn't a pipe where the medicine cabinet will someday go. But you may have to watch your plumber.