I like the 18" mirror with the 24" vanity in your drawing, I think the scale is pleasing. The mirror in the guest bath almost looks too big when I compare with the hall bath. Lights - can you buy a 2light for the hall and the 3light fixtures for the other two bathrooms? If you wanted the 3light for the hall bath, I would probably get the 24" mirror. Have you tried drawing a light fixture to scale above these mirrors to help with your decision? You could print your drawings out and draw on the paper how they would look if you can't find a way to do it in the drawing program. In your powder room drawing, the mirror is not centered over the vanity. Is this intentional? If so, why?

Mari, we had a similar situation, but if the room is being used for grooming I would really recommend going with sconces even if you don't think you have the room. According to all the advice from the experts I googled, we didn't have room for sconces either. I really wanted sconces since everyone said they gave much better and more flattering light (and who are we kidding, we need all the help we can get). I got a big sheet of mirror and had the sconces mounted on the mirror. Here is a pic of our mirror. Our vanity is 22.5" wide. If this really isn't an option for you then I would get a framed mirror with dimensions slightly smaller than your vanity width. If you are getting something prefab youre probably going to be looking at 24"x36"

If you want to use a medicine cabinet I would probably stay with the 24 inch and use side lights. You'll need to keep in mind that a bar light above the medicine cabinet will need to extend enough to clear the depth of the cabinet, so finding such a light may be difficult. There are cabinets that have built-in lights so you may want to look at those. Check these out. If you choose a cabinet with built-in lights, I'd go with a wider than 24" cabinet. Other option: keep the light fixture you have and put in a framed, or unframed mirror that is a little wider than the lights. Here are some visuals.

I think a 3-light will probably work. A 2-light in a powder room, maybe, but you'll want something like 3x65W equivalents of light to get ready in the morning. As long as it's not wider than your vanity, a little larger or smaller than the mirror is not likely to be noticeable.