You don't have to match fixtures in different baths. In fact, you probably couldn't find a designer to recommend that. Builders do it because it is cheap and easy to put the same products everywhere and in every unit. As for the size, it is clearly a matter of personal taste. However, most women find it practical to have the light coming in at an angle rather than overhead.

I prefer two mirrors as well because I think it gives a more balanced look and it provides defined spaces, his and hers in our case. Your statement about wanting to make it so that 3 people can use it is what has me curious though, is this a what if type scenario or is a 3rd person actually going to be in there all the time? The reason I ask is because DH and I used to make choices based on the what if's a lot. We'd spend the money or compromise on what we really wanted only to never have those "what if's" materialize. We bought a crew cab truck so if we needed to we could tote people but never once did anyone sit back there. We turned a room (that could have been my craft room) in to a guest room and in 10 years it's only been used a handful of times and an air mattress would have worked just as well. So if you want the big mirror because it will actually be used in the context you're planning for, great...then there's a need and that would be the more functional choice for your situation, but if you're just thinking well maybe 3 people might need it, then don't focus on that and do what is more practical for your every day living. KWIM? (And Ida is right, about the two lights)

Thank you so much for these really helpful comments. @kandrewspa, Interesting suggestion about changing the sconces next to the mirrors, and I'll consider it although I do really love the ones I've got. Re the ceiling fixture, I do have one but unfortunately it is primarily an exhaust fan with just a small LED light, which I forgot to turn on when I took my photos. I need that fan, and there isn't room for an additional ceiling fixture. That's why I wanted to put in the lighting over the mirrors, to augment the sconces at the sides. I do already have both categories of lighting on dimmer switches; forgot to mention that. @JustMe, for up-facing sconces to go above the mirrors, do you mean something like this or this from Justice Design, or this one from Capital Lighting, or this one from Swiss Passport? Would you place them above each mirror, or between them?

One thing to keep in mind if you're going with Theresa Peterson's recommendation is location of any power outlets, if you have any - need to make sure that they remain accessible. You can always hang the mirror(s) above them, but then the mirror(s) may be too high. I have that issue with my currently remodeled master (this is the "before" photo) - because of the power outlets, and because each mirror needs to be centered with its respective faucet, I cannot do wider mirrors (even though I have the space, theoretically):