The plumber will calculate the btu load and length for each segment of pipe, and size the pipe in accordance with local code requirements. It will start out large and decrease in size as each fixture branches off. By the time it reaches the gas logs it will probably be down to 1/2" pipe.

@ramona1976 is right, the BTU load has to be calculated for each section and sized accordingly. That's not possible with the info given.

Here's an off-topic opinion though: You are building a new house in the South East US. I'm guessing an a rural area based on the lack of NG and the barn. I'd diversify energy sources, put a lot more emphasis on solar PV and dial waaay back on the propane. This house is going to be around for 100 years or more. Build it for the next 100 years, not on ideas from 60 years ago.

Start with a hybrid heat pump water heater, and heat pump for the main house. Maybe a heat pump for the barn as well. Keep the propane for the generator, gas logs and stove if you want.

Put panels up to supply much of your electrical needs.

Propane is expensive - and it's been unavailable before. Don't put yourself in that position.

Thanks for the comments. Yes, we are using "dual fuel." Both the house and garage will have heat pump/propane furnace. Natural gas is available in town (about 3 miles away) but we are just beyond it's reach. Solar has some appeal. We'll definitely keep that in mind.

The tankless water heater is in bedroom #2?

No, it's in the crawl space. I didn't take time to describe the entire house design - just trying to convey the distances and the BTU's involved.

Provided you're in an area that has a building code, what is most important for you, as a homeowner, is to be sure the proper permits are pulled for the installation of the tank and gas piping, that a properly licensed gas fitter performs the work, and that inspections are performed.

The piping will be inspected. The installer is a long-time tradesman in the area, known by the propane company, and operating under the permit of the HVAC contractor. I think all is good from that perspective.

SE US is a big area. I am in Raleigh and have a nearly all-electric house. Just NG for cooking and fireplace. If I had to do propane, going all-electric is a no brainer. But rural could favor a back up source.

But all this propane infrastructure is awfully expensive for something that nearly always costs more than good old sunshine.

I have an 18k solar system that covers the house and then some. Just food for thought. Had dual fuel in the last house and then realized the infrastructure just isn't worth it....

One of the things that really jumped out at me was the tankless water heater.

If I were designing a house - in this location - I'd plan on a hybrid electric unit. It's the least expensive to operate, and you avoid some issues that tankless heaters have. They also play very well with solar PV.

Hybrid electric produce some noise (as much as a refrigerator) and they need some open air (garage, basement, laundry room - or can be ducted). So you do have to incorporate that into the design.

If I had to do it over again, I'd install a hybrid electric instead of my tankless.