What's important is the distance between the lights, regardless of the size of the mirror or vanity.
A: It’s not so much the size of the mirror as it is the distance between the task lights. The optimum spacing between vanity lights is 36 inches. This means that the fixtures would be located 18 inches on either side of the center line of the sink. If you are using a regular mirror, then it would be 30 inches wide, allowing 3 inches of spacing between the edge the mirror and the edge of the backplate for the vanity lights. There are companies like
Robern that offer mirrors with built-in side lighting.
If it were my bathroom I would run a mirror along the entire width of the vanity. I would then install a pair of fixtures on the mirror. If you'd like to leave the mirror surface uninterrupted, you could hang a pair of pendant fixtures in place of sconces.