My one bathroom has 2 fixtures that can take 100 watts each. I stocked up on halogen (72watt?) that are comparable. It is very bright in my 6.5x9' bathroom with light gray/blue walls. Very bright for husbands shaving. Now with the powder room, you wont need that bright. I'd think a couple sconces with lower wattage such as 40 or 60watt equivalent lumination would be fine. How big is your room. You can always put lower wattage bulbs in a fixture. I have dimmers, but find that i seldom use them. I like going into a powder room that isn't like a surgical suite. I like lights to add ambience with twinkle and drama, and a powder room could be a great place for that IMHO.

Thanks all or your input! I'm going with two. Two is what I originally thought but then fell in love with the PB Astor Double-Width Mirror (a 58" X 40 beveled mirror framed with beveled mitered pieces of mirror). It's the only large mirror that fit the idea I had in my head for how I pictured the space with "one large mirror", but it seems to have been discontinued on PB and there is no duplicate to be found (I even went to a glass store to see if they could make it and they cannot). So two mirrors it is. My base tile that was on backorder (giving me more time to work through all this) gets installed tomorrow so now I just have to figure out/order the lights....either overhead, or sconces that would be squeezed in. There will only be 6" of space next to the right-side mirror so I'm leaning toward overhead.

Aaron- Here is a useful explanation from Fine Homebuilding of what I was talking about in my comment above: Avoid shadows at the mirror Vanity lighting is the most important task lighting and can make or break your bathroom experience. A light, or row of lights, above the mirror to light your face creates shadows under your eyes, cheekbones, nose, and chin that make you look tired, heavier, and older. Cross illumination is much more flattering. By placing vertical lights at eye level on both sides of the face, you eliminate shadows. Generally, use lights with some sort of shielding here, whether it’s frosted glass, acrylic fabric, or some other material. Especially if it’s a fluorescent bulb, this softens the light and adds warmth.

Here are some examples of full mirror bathrooms. Is your vanity inserted between two walls? Personally, I like the full height mirror if no wall can be seen on the side of the vanity. https://www.google.com/search?rlz=1C1CHBF_enCA770CA770&sxsrf=ALeKk00ZnBFpKlIrLygCR5pM7VJ7qLc9vA:1590888282656&source=univ&tbm=isch&q=vanity+with+full+height+mirror+images&sa=X&ved=2ahUKEwix3NvX-NzpAhWkdt8KHYq5AIUQsAR6BAgCEAE&biw=1898&bih=902#imgrc=-yII9zb3QV2FfM