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)

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.

https://www.houzz.com/hznb/photos/contemporary-tiburon-home-contemporary-bathroom-san-francisco-phvw-vp~2864264 This is the look we are going for

You can have mirror cut to any size by a glass shop, but if this is a bathroom that is used every day for getting ready in the morning (not the powder room), if I were you I would engage an electrician to have sconces on either side of a smaller mirror. The light you get from sconces is much, much better than overhead lighting. This isn't a big job and if you pay for it once you'll have it forever. A decorative mirror will look better than a big mirror too. If you do go with a big mirror, frame it in some type of molding so it doesn't look like a tract home mirror.