72" is a really nice size to share. The extra counter space doesn't hurt either. It will still give you plenty of space on either side.

72" will not only give you tons more counter space and elbow room, it will give you more storage. I like lots of drawer storage in a vanity. Our double vanity has 3 separate banks of drawers - so drawers, sink/cabinet, drawers, sink/cabinet, drawers. Our is 84" though so not sure how much you will be able to fit into 72" ... but would recommend fit what you can.

Why would you even consider the smaller vanity? What would you do with the extra 12 inches of empty space? Go 72 or larger.

Adding this photo so you can see what fits in 7 feet of vanity space. It sounds like your intention is to have open space on either side of your vanity, if so consider the difficulty of cleaning there if it is a narrow space.

Thanks so much for all your comments! I'm always so happily surprised by how helpful fellow houzzers are...

Re: positioning of vanity, I'm attaching a Houzz inspiration photo for or bathroom - we chose a similar furniture-style vanity (ours is more walnut) and will have the shower at the end of it. The only differences really are that we have our toilet under the window (unforch no way around that) and I think we'll have a kitty-corner glass shower with no wall built up (if that makes sense). Also, we chose a Bertch vanity because we assume it's cheaper than going fully custom and 72 is their biggest size.

So, I'm now choosing between centering it on the wall (and putting hand towel holders on each side with a waste basket closer to the entry door - would 12 inches be enough for that?) or sliding it to the side, to be flush with the shower, which would give me the full 24 inches next to the door to put a waste bin and possibly get a narrow tower in... Or, like, a side medicine cabinet - I'd love to get round mirrors for above my vanity, but am nervous about loss of storage...

Anyhow, thanks again for all your advice and look forward to any other thoughts on this!

Does anybody have an opinion on the max size of the sinks for a vanity of this size? We can go deeper on our countertop (up to 28.5"), but I am concerned about the rectangular sinks I would like. My vanity will be 70-71" wide.

Center your vanity on the wall. It will look better and be more symmetrical. Add sconce lighting--sconces are way better looking than over-the-mirror lighting. You can use a double on each side and a triple in the middle between the two mirrors.

Concerning your vanity/sinks--make sure that the sinks line up with the cabinets. Center of sinks with center of doors. No need to get super big sinks. You need to decide what's more important to you... large sinks or more counter space. I'd pick counter space and medium sized sinks.

Getting the lights + sinks + faucets + mirrors all centered with the cabinets is very important. Many dilemmas on Houzz when people didn't pay attention to these details.

Do you have a picture of the vanity yet?