Mirrors tall enough for you to see what you want to see, and wide enough for the same thing.

A 6-foot vanity is not very wide...have you considered a full width mirror by as tall as the tallest person in the family?

What's your reasoning for going with two separate mirrors? A single mirror is more comfortable to use (you never move out of the reflection space) and makes a room look larger.

With a 6' vanity, I'd want a 6' mirror.

A 6-foot vanity is not very wide.

I have a 4' wide vanity and no desire for anything larger. One sink, drawers on both sides, 4' mirror above.

Mrs. Pete, I think Mr. Virgil's comment of a 6' vanity not being very wide is meant in the context of using 2 mirrors (6' not being truly wide enough to really accommodate 2 separate mirrors), which is why he recommended a single one. :-)

That's how I took it, anyway, lol. Not that he had anything specific against the size of the vanity, in and of itself.

OP -- Have you been shopping for mirrors yet, to get a feel for the average width of them, to see if your space could handle 2 separate ones? Do you have a way you could tape off some sizes on a full-width mirror, to see what you like and how the 2 mirrors would be spaced?

Where will your lighting be? Over the mirrors? Ceiling can lights? Wall sconces on the sides?

You wouldn't happen to be open to sharing the floorplan, would you? (I know, a dangerous request on this forum, lol.)

Two lavatories in a 6-foot counter is pretty tight...just ask Bob! :-)

Bob knows Ms. P likes her four-foot counter and single lavy. but Bob knows that's not what the OP asked...

Sometimes I'm glad ole' Cowboy Bob is my good friend...!

If you did 2 24 inch wide mirrors with 3 inch trim around the mirrors, you would have 4 inches between mirror trim and 4 inches on each side of the mirror trim. Not much space between, but enough for some wall color!

I thought I liked that look also with sconces on each side of the single mirrors. However, one counter is 5' and one is 3' and meet in an 'L' with a countertop cabinet. The biggest problem besides 2 different sized mirrors was that I wanted them going up to 8' to add height to the area and match the cabinet and window height. It was much easier and much less expensive to get an expanse of mirror from the decorative backsplash all the way up to 8' in the width of both counters. It looks great and I have the sconces on top of the mirror surface and it looks great. In addition, it gives great light! You can also have the mirror edges beveled. Some have it framed. I have neither!

No one seems to actually answer your question.

Mark....what size would you suggest if the two mirrors are round? Thanks!

36"

Two 36" round mirrors are going to be very close to each other, perhaps touching, which I think would look awkward. How about oval mirrors?

Here's a 66" double vanity with 21" x 30" mirrored medicine cabinets:

painted cabinets · More Info

I think these proportions are good, but they were determined by drawing the elevation to scale.

Thank you Mark and Diana. Perhaps 28-30" round mirrors would work?

They may work, but you'd need to make sure that they are tall enough to see what you want to see.

To see your whole body in a mirror, it needs to be at least half your body height.

(I knew I took those physics classes for a reason)

Diana...Here is link to the 72"VANITY

https://www.homedepot.com/p/SUDIO-Jasper-72-in-W-x-22-in-D-Bath-Vanity-in-White-with-Engineered-Stone-Vanity-Top-in-Carrara-White-with-White-Basin-Jasper-72W/314491746

Per your suggestion here is link to a black matte 24 x 36" oval mirror that I just found.

https://www.homedepot.com/p/Umbra-Hub-24-in-H-x-36-in-W-Oval-Black-Modern-Mirror-1006044-040/30520501

I am thinking of keeping the vanity's included nickel hardware and adding matte black faucets.

Do you think the mirrors 24" x 36" ( hung vertically above each sink) is a good proportion for the 72" vanity?

Without seeing it on an elevation, I can guess that it would be good. According to Mark's rule of thumb, unless you're over 6 feet tall you will be able to see enough with a 36" tall mirror.

If you're buying a ready-made vanity with a counter top, I see that you cannot butt it against the wall, If that works for you, then go for it.

Of course, if I want a taller mirror could do a rectangle....but thought the round or oval looked good!

Thanks for pointing out the "butt" issue. Vanity will be centered on an 84" wall so will not be butted.

Thanks to both you and Mark for your advise!