So many questions, I don't know where to start. :lol: Just kidding.
For the size of the shower/toilet in the bedroom, think Superman sized telephone booth, and then go smaller. I'm just shy of 6' tall and weigh about 165 and I can't take a shower in the booth without banging my elbows on the walls. In fact, after trying just once, I've never used the shower in the room since. I always walk downstairs to the public shower, since it is a bit roomier. I also find that the water pressure is a bit better too.
Now, back to the in-room shower. The shower head is not fixed, kind of like a water-pik shower, you can lift it off the hook and the flexible hose allows you to move it around as needed. The same is also true of the public shower. There is a plastic curtain that you snap down around the door to keep the water from splashing/running into the room. And there is a 2 to 3 inch door sill that prevents the water from running out of the shower at the floor level. However, when you open the door, it is not uncommon to have some water drip off the door and onto the floor.
Amtrak recommends, and most people that I've heard from do so, putting the toilet seat and cover down and simply sitting on it while taking your shower. And yes, everything in the stall gets wet. There is actually a plastic cover over the toilet paper to keep it from getting soaked. Amtrak does provide soap and towels, although you may prefer your own, assuming that you want to carry it. The towels aren't thick and luxurious by anyone's standards. Amtrak does not supply shampoo and conditioner though, or at least it's not out for you to grab. It is possible that the attendant might have a small supply for someone who forgets it, but I wouldn't bet on it.
As for the water, each car is equiped with a water tank, a waste water tank, as well as a hot water heater. The supply of water isn't unlimited though, and therefore Amtrak recommends taking short showers. And during the morning hours when most people seem to take showers, you may find that the hot water runs out if you take too long of a shower and too many other's are also showering. The train does get rewatered during its journey.