Our Powder Room is approximately 4' x 7' but with only an 8' ceiling.

Originally it had an over-the-mirror fixture with two bulbs. Actually that was sufficient lighting. When we updated, the original fixture was replaced with two mini pendants on a dimmer - they were placed on opposite sides to eliminate a mirror shadow. We also added a small canister above the WC, also on a dimmer. Additionally, we built a niche and placed a puck inside. No tile in the space, wall covering all around with a white ceiling.

The lighting is very sufficient with the pendants at full power. They are low-voltage halogen but I don't know the wattage. Tend to use the others for accent.

I believe a single 100 watt will add sufficient illumination however the shadows will be controlled by the distance from the wall and height above the person using the mirror. Perhaps you can do a test with a table lamp held in various postitions. Personally, if in doubt, I would install can lights rather than a sconce so as to not take away from your chandelier. They could be on the same switch and with your ceiling height guests would not even know they were there.

Here are some photos of our Powder Room. Good luck with the remodel.

Here is a link that might be useful: Powder Room