I imagine there is a design principle that would serve as a guide as to how long a shower can be without looking odd. Unfortunately, I don't know where to find that information! Maybe someone else on this forum can help with that. My current linen closet (in the hallway) is 24" wide but due the framing of the door, the opening is only 17" wide. It is not very functional, IMHO. I recently built a new floor to ceiling closet in the master bedroom with sliding shoji doors (still waiting for the doors to be built). I included an 18" wide linen 'closet' and the space is totally accessible - much better design. Additionally, the shelves are on drawer runners because the closet is 24" deep and being able to slide out the shelves makes it much easier to get to that towel in the back! So, I suggest you take a good hard look at your linen situation and try to picture yourself actually getting things out and putting them away. From there you should be able to come up with a good solution. From someone who is on a mission to enlarge her house by getting rid of stuff!

I do like the idea of eliminating the upper cabinet and replacing it with a mirror. I prefer the dark counter, especially since you already have it and it's perfectly fine. Instead, while I can't see the light fixture, I'd put the money into lighting, esp around the mirror...either above or to the sides or a pretty pendant in front. I think the darkness you feel is more from a lack of light than from the colors in the room. The overhead is good for general lighting, but lighting around the mirror makes you and the room look better.

First, I do agree with @emilyam819 that it’s important to budget for the things you don’t see. The materials needed to properly build and waterproof the shower are not cheap and shouldn’t be skimped on. Also, shower and tub fixtures require parts that go behind the walls. Be sure any plumbing fixtures you buy include all that parts necessary for assembling them. For example, check to see if sink faucets include hoses and a drain. Often they don’t and you have to buy those items a la carte. Also, to help with suggestions, it would be helpful to know if you prefer wall mount, four hole, three hole, or single hole. Also don’t forget to budget for your sinks and countertops. I definitely would stick to tile only in the shower and on floor from a budget perspective. More tile looks gorgeous but adds up in terms of both labor and materials. Also, make sure you talk to your contractor about exactly how he installs and finishes tile (including the prep steps he does before he ever puts the first piece down). Read up on Houzz on proper tile installation - you won’t become an expert but you’ll learn enough to spit red flags. Just search the advice forums for tile questions - you’ll be horrified at what you see and it will help you be cautious and ask good questions. Love everything you’ve picked out and I agree that vanity is a steal. Personally I would choose all gold. Mixed metals can look great but I’d be uncomfortable attempting it without a pro advising me. As for the floor, I think it’s important to consider the other flooring in the house, especially in the area where the bathroom is. I wouldn’t do the wood look tile if you have hardwoods elsewhere. And if there are bathroom floors elsewhere that you plan to keep, I’d choose something relatively similar for this room. Or if you plan to rip up other bath floors later, consider something you’d be willing to use on those floors as well. The idea is to not have the floors in all the rooms of the house feel like a quilt - all different, with no relationship to each other.

A few thoughts: - We wanted a black curb and pony wall top but could not find anyone to do it for a reasonable cost. Black on its own is available in 6" widths at Floor & Decor type stores, but not with matching vanity, and that was too wide for our curb and wall. We gave up in the end and did the vanity and curb/top separately in white marble-look quartz and actual marble, respectively. If you are doing multiple bathrooms, though (I think you are?) you can work with a fabricator for that and use materials for multiple baths, so I would try to coordinate the vanity/curb materials (if not the surrounding aesthetic). - I would do a neutral colored vanity with popping red accents rather than a red vanity. Less to change later and I think you could get a lot of red in there with knobs, accents, wall color, etc. without committing full-on to a custom red vanity. - At GC and tiler's rec, did not tile the front of the pony wall--but we also didn't wrap the tile onto the wall, and it makes sense to if you are going to do that. It you have a good tilesetter, he/she should be able to miter cut the edges to wrap the corner cleanly, and I'd personally prefer that look. Depending on the tile you choose, they may also make trim pieces designed for outside corners. I like your tile, but it's going to make for a very dark room. I actually really like the variegated blue tile in your rendering--I wonder if you could find something like that to introduce a little variation? If not, I would only take the blue partway up the wall and would then switch to a lighter color from that tile line to go the rest of the way to the ceiling in the shower. -Niche on the tall side wall or else on the shower head wall.