It's not uncommon for them to be different sizes, and you have them delegated properly...the woman gets the larger vanity. That's a serious statement in terms of the larger vanity having the "proper" lighting, storage, etc, around it.

They can be slightly different, the difference doesn't necessarily have to be substantial. If there is a slight difference, the essential design of drawers and doors can remain the same if you choose, and the proportions can simply be skewed.

With the vanities being custom, I'll often times make the man's vanity taller. So yours can be shorter but wider. His can be taller but narrower. You both get similar storage, etc.

I wouldn't go too small with the width of his vanity. 36" can be minimal depending on the setting. Example, if his vanity has solid walls on each side, is might appear to be shoehorned in and it can accentuate the smallness of his space. If there is free air on one or both sides it can have a more open feeling, or a glass wall on one side can open it up visually, etc.

He needs to be happy with his space. You need to be happy with your space. Then use whatever space can be freed up for a larger shower if that makes both of you happy.

With the shower, your shower right now has decent dimensions. So think about the additional shower space and why you want it, and if it will accomplish those objectives. Will the additional 6" of space in the shower get you to that comfortable two-person shower if that is your goal? Or will it be a cramped two-person shower? With a bench you'll be sort of cramped even as a single-user shower. Without a bench? Etc. Will you realistically be showering together regularly, or just occasionally? Or never?

If the added space in the shower doesn't fully and completely achieve your shower goals, then it might be best to leave him with a larger vanity.

very common for them to be differently sized or shaped.

We had different size custom vanities built for our master. My husband and I each chose the drawer and cabinet configuration that we preferred and we're both happy with how they work for our needs. I think that they complement each other and I think you'd find the same in your space.

Hers...

His...

I agree with Mongo and would not reduce the size of your husband's vanity to gain another 6" in your shower. Functionally, that wouldn't be a trade-off I'd be willing to make. Also, I think that a 36" vanity beside a 54" shower would look disproportionate. I'd leave the shower at 44 x 48 and go with 42" for his and 48" for hers. I'd have no concern about it looking like a mistake.

Good luck and I hope you'll post pictures as your project progresses!

My vanity was a double size (with a single sink) so it was longer and not as tall. It's along the left side of the picture. My husband's was around 36" tall and about 40" wide if I remember correctly. This was in our previous house.

My master bath has his and hers vanities on opposite walls. Hers next to large tub and his next to shower. His is taller than hers and not quite as wide. Hers has an ornate black framed mirror and his a more masculine wide, plain black framed mirror. I am going to be changing the tops to matching granite or quartz and would like to have a top vessel bowl sink on hers and an undermount sink on his, both of same material. Would that be a design "no-no" and would it be a negative for future resale? I would love the personalization of having a different sink but don't want to negatively affect our resale in the future.

It all depends on the size of the bathroom and the orientation and placement of the vanities. It could look fine, or it could look like you skimped on one and look odd. A dimensioned floor plan with the proposed layout would help in your getting more informed opinions here. I tend to go with uniformity and symmetry in things like this, but that might not be an issue based on the layout options you might have available.