Just to be different, I like having a three-way hinged mirror so that I can see to cut and style my hair - not that it gets much in the way of styling most days, lol.

The medicine cabinet we installed in our master bathroom has that set-up, and we will be replacing the single, plate-glass mirror in the guest bathroom with something similar.

I think the single mirrors definitely are more decorative and you then have the option to add an additional element in way of the frame you chose. You may also have room to add a single sconce between the mirrors for an added level of light or some other decorative item. Maybe have space for a counter garage. I can't help you off hand on the mirror sizes. Mine are packed in the garage of the other house. To get an idea go to the gallery of the decorating forum and look up the bathroom eye candy thread and there are literally hundreds of photo's. You should be able to get some ideas from there by looking at idividual mirrors and how their size relates to the vanity/bowl size. You can also google vanities with matching mirrors and that will give you specific sizes. Good luck.

I personally prefer 2 mirrors, one over each sink. With a vanity that long you could put a wall cabinet in between them. The mirrors in our master bath are about 27" wide x 33" tall.

If you have two medicine cabinet mirrors and hinge them on the outside, so they swing toward each other, you'll be able to see the back and sides of your hair. I proved this to myself by covering up the center section of our tri-view mirror.

We wanted two mirrors for our 90" vanity. We felt one mirror would overpower the room. Two makes the space cozier.

We agonized over the size of the mirrors. We went to a lot of Open Houses and home improvement stores for ideas.
Our ceilings are 9' and so we ended with mirrors 30" x 38" (outside dimensions).
Even though our two sinks are set 13" from the outside edge of the countertop, with a mirror centered over each sink, there wasn't room for side sconces. So we have a sconce over each mirror.

Think about the ability to view the sides and back of your hair for blow-drying, trimming or whatever you do. If you get a tri-view, you will have to bend in quite far to see a limited area of your head. The larger the mirror, the easier the view.
I suggest going to a store or showroom, opening the cabinet and checking to see just how the inside mirror works in allowing views of back/hair.
I have the two largest mirrored Roberns, butted side by side. It's easy to do the sides and back of my hair.
I also have a wall hung hair dryer so it's a relatively simple, time-saving, frustration-saving, process.
For me, if my hair looks ok, everything else is better!
Don't wanna stretch in too far. Hurts my back!