" What is the right size hot water heater for a house with my specs and how should this have been approached initially?

An on demand system combined with a storage tank or at least 2 50+gallon tank heaters and preferably a 3rd smaller heater near kitchen/laundry.

"The second is more practical: is there anything I can do now that will enable me to take long, hot, multi-shower head showers if I'm so inclined?"

A point of use booster can likly be added if shower backs up to a closet or is supplied from overhead. If hot water loops above floor under a vanity or garden tub located upstream from shower,booster can be installed there. Some if,ands and buts with doing it,just have to have possibilities explored.

My 1700 sq. ft house has a Heliodyne solar water heater with an eighty five gallon storage tank. In summer I have 85 gallons of 185 degree water in that nearly every night. It is backed up by a Takagi TK1-s on demand hot water heater. I have a big soaker tub, two showers and three sinks. With the on demand H2O heater you will wait a couple extra minutes for hot water to flow but I can run 2 showers simultaneously or the the soaker tub and never run out of hot water. The solar hot water arrives almost instantly. I am in Wisconsin so my solar H2O heater doen't produce much from November through January but the rest of the year I hardly use the on demand.

If your other water usage is far from the tank you might want a recirculation system on a timer or some smaller on demand heaters located under the sinks if they are located some distance from the main 85 gallon tank.. My farthest bathroom sink is located 50 feet from the solar tank/tankless so it takes about 3 minutes to get hot H2O so you have to plan
ahead a little bit. With an on demand heater and/especially the the solar heater you are better off to just let the hot water run even though you are not constantly suing it, otherwise the on demand shuts on and off and you will experience some shorter delay time before you get hot water.

If you are in a southwestern state like CA, AZ, NM TX with some supplemental heaters near the small demand areas and you should have no problems whatsoever.

It sounds like your GC did not have any clue as to what he was doing concerning your hot water needs.

A correctly sized solar H2O heater is the only way to go if you need plenty of hot water. If you use a lot of hot water the solar water will pay off in about 6 years. There is nothing like free hot water, it is one of your bigger energy expenses.

You should have consulted with a plumber before planning the entire bathroom. Water wasting fixtures like multi head showers WILL run any normal hot water production system dry in a very short period of time.

NASA Scientist Jay Famiglietti says CA will run out of water very shortly.

I have a Triangle Tube Series 50 (45 gal.) in a 2,000 sf house with 2 bathrooms. My plumber said I should put in a smaller unit and I wanted a bigger one. We compromised on this unit, because he said a bigger one would consume a lot more energy.

I take a 25 min shower with a 5 gpm shower head, and I get enough hot water from this unit, but it took some doing. The first thing I would suggest is to turn the temperature on the water heater all the way up. We have an L-shaped ranch with the water heater on one end of the house, and the master bath on the other (over an unheated crawl space). So we need to keep the water very hot to get enough hot water at the bathroom. The hotter the water is, the more cold water you'll mix in, and the longer your hot water will last.

Despite this, I was still running out of hot water after about 15 min. As my shower valve doesn't have a volume control, I put a small valve on the shower arm which allowed me to slow down or shut off the water when I was soaping myself. This allowed the hot water to recover a bit before I finished my shower.

The real breakthrough came last summer though, when I had a new boiler installed. The installer told me that the circulator pipes between my boiler and water heater were only 3/4", when they should be 1 1/4". He also set up the new boiler so that the water heater gets priority over heat to the house, and the boiler automatically heats up to 180 degrees whenever the unit calls for more hot water. Now I have plenty of hot water for a 25 min shower followed by a shave, even without turning the volume of hot water down during my shower.

I think you should be able to get enough hot water out of this unit provided you use it to its full capacity. You may need to have someone do some work on your boiler though. One rule of thumb I live by, is to never let a contractor tell me how much water or hot water I need.

Aside from this, do you have separate shower valves for your 2 shower heads, and are you satisfied with the water flow? Many newer shower valves limit the flow to 2.5 gpm or less. You can check your actual water flow with a 2.5 or 5 gal bucket and a stopwatch. If you're getting enough flow out of the shower arms, but not out of the shower heads, there are some places that sell shower heads with easily removable flow restrIctors.

This is GREAT. Thank you everyone! I'll try turning up the temp and have my (new) GC check out the boiler setup before resorting to anything more drastic (but good to know some of my options!).