I think different approaches work for different people. Our bathrooms are small (master is 6' x 10', hall bath is 6' x about 12'). In the master bath, we have a 60" vanity that abuts the wall on one side. Over that vanity, we have a full-size mirror (as described by palimpsest) with a 24" LED fixture above each sink coming through the mirror (love that look). In the hall bath (used by DD, and also serving as guest bath), the vanity is in a 55" alcove. We chose to use a free-standing 48" vanity and a "free-standing" decorative mirror mounted horizontally, with the same LED lights hung vertically to the left and right of the mirror. The light from the LEDs (George Kovacs) is terrific, much much better than the light we got from the recessed lighting and over-mirror lights in our previous home.

Your sconces are too high. They should be at eye level. The shades are too wide, leaving you with a mirror only the width of the sink, which will look odd. The time to plan all this out was before the wall paper was hung. I would bite the bullet and do it right instead of trying an imperfect fix.

I don't know that I would try to hang two mirrors in such a small space. I think it may just accentuate the fact that your vanity is too small to have two sinks in it. I would get one large mirror centered with the entire vanity. I wouldn't pay for something custom, there are a million large mirrors out there.

In terms of proportion anyway. But you're right, round mirrors are a little different. I wouldn't get one any larger than 40". Also, if you get a frame you may want to match it to your hardware. Here is a photo of a round mirror with a size proportional to your vanity. Of course the placement of the mirror also depends on how tall you are. I wouldn't get a round mirror any larger than this.