Don't know if there is a "standard" size. We had all our bathroom mirrors custom framed as we needed sizes that fit our own individual needs. In the powder room that meant a mirror that would work with a 7' ceiling and for dh who's 6'3". Not easy! The mirror is 26" x 38". The vanity is 52" with sconces. In the kids bathroom we needed mirrors that would also work for much shorter children AND tall parents or guests. Here the vanity is 84" with 2 sinks and lighting above the mirrors. The mirrors are 24" x 36". The ensuite bathroom has a vanity that is slightly shorter but also with two sinks and lighting above the mirrors which are the same size as in the kids bathroom. All mirrors have frames that are 2 1/2" or 3" wide. I like the look of mirrors with wider frames but that wouldn't have worked with the function we needed, i.e. work for short and tall people.

I love the look of a well done tower. I see no reason for multiple heights in vanity below. You have 108 inches in niche to play with........ Left to right, if you want the effect of your third picture: 3' "post", 30 inch door base, 3.0 post, 36 inch 3 drawer base, 3"inch "post", 30 inch sink base 3.0 post. is 108 inches. Tower above in middle 36 wide, 12 inches deep max with two doors, , and whatever inches high to ceiling. No raised counter, no lowered counter : ) Easy to work with three inch increments for all and stay with three drawers on the drawer base.. Use bins and divvy up the space. Note: Build a "base' for tower, use a 6 inch splash, and WRAP that tower base in same surface as counter top. The higher splash is more elegant, and it will keep wood OFF the counter surface. Yes you are fine on the 36 inch drawers. Yes, that's the width of a lateral file with a fraction of the weight inside : ) it's fine. Door bases are a black hole of junk behind, and I like the "weight": at the middle. As to "wrapping " that tower base.......here's what I mean. Not for your style just for tower base. With a six inch splash, pic below That arrangement will give you 36" wide spaces for mirrors/ frame included over each sink by whatever height you want. ,

@ptsuruda It was $1850 when I bought it, but it looks like they've raised their price to $2000. Still not a bad deal considering it includes shipping, you can have them design it exactly to spec, and they use walnut plywood, which is not cheap. https://www.etsy.com/listing/881307908/bathroom-vanity-cabinet-mid-century It did not include the top. I bought a marble top for $900 installed with sinks. You might end up paying more for quartz since it's a synthetic material. Best thing you can do is go to different stone shops and get quotes-- they vary widely. "Kitchen and Bath" stores will charge way more and will just go to a stone shop themselves, so avoid those.

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):