For more than 20 years we used a master bath vanity that was only 58" long. The two sinks were the self-rimming variety that ate up extra counter space, probably 2 1/2" per side for a total of ten inches. So what we were really left with was about 10" of counter space divided between the two ends and the middle---ok for the tooth brushes and toothpaste but not much else. Undermount sinks in this amount of space, and there are many beautiful ones 18" or less, would have given us a total of about 20 inches of space but we just never got around to making the change what with orthodontists and saving for college for the kids. We did have a bank of 4 drawers down the middle of the vanity that was about 6" wide in the interior and amazingly enough we did find this sufficient for storage for what we really wanted near the sinks. The major issue for us really was one of elbow room since the vanity was in an alcove but generally DH and I weren't brushing our teeth at the same time. Overall I would say that with 60" of space and undermount sinks it would be a real plus for a resale as long as elbow room didn't seem to be a problem.

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 only have 2 children, a 14 yo girl & 17 yo boy...and we have 3 bedrooms & a playroom upstairs (master is down.) My son's room has its own bath, but we actually added another door so it also opens to the playroom. So glad we did this! Our daughter has a Jack & Jill with the guest room. She has her own very large vanity with a sink and drop down counter area where she can sit & apply makeup. Her large, walk-in closet in is this dressing area as well. There is a door to the shared toilet/tub area, which of course connects to the vanity area of the guest room. The guest room vanity is much smaller, a single, and the guest room closet is just a small walkin in the actual guest room, not the vanity area. If we had 2 teen girls sharing this set up, the one in the guest room would definitely get the short stick! Since we custom designed our plan & knew we were only going to have the 2 children, our set up works just fine. Dd was 8 when we built, and we knew the teen years were coming. Teen girls just have so much crap! Personally, if I were you...I'd make the two older ones involved in the process. I bet they would prefer sharing a J&J to having a hall bath. Make sure they understand you are not getting involved in any fights & that they have to figure out a system. It will be good practice for college where many dorms have this type of set up now. The little ones can have the hall...and when your littlest dd becomes a teen, the other girls will be adults & she can have the J&J to herself!

In kids bathrooms, size the mirrors proportionately to the sinks and cabinet doors below. I'd do a rectangular white frame to match the cabinets. They can be replaced with larger when desired. As for hardware, I'd want it to be chrome. You didn't ask, but flat panel drawers are ideal in a bathroom. The shaker flat edge will catch lots of dust and stuff and be harder to clean as well.