It is about flow. To get flow through a pipe, you need differential pressure and pipe size. Bigger pipe means smaller differential pressure. And the appliance needs a certain amount of pressure at the input to work.
Gas in this country is regulated to 7-11 in Water column before entering the house.
See here, a small online calculator
https://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html
The only thing, you need to know the units (inches of water - pressure) to use the calculator.
On a Pool heater or generator, the "smart" thing to do is to run the pipe to the heater at 2 psi and then put the regulator, 7-11 in-H20, right at the "appliance". You cannot do that inside. If this heater can be outside, this is eezy-peezy. Run the pipe at 2 PSI and put the regulator right in front of the heater.
How many BTUs/per hour is the heater rated? Divide by 1000 and it needs that many cubic feet per hour of natural gas.
The biggest one I've found at Home Depot is about 200,000 BTU/hr.
So, 200,000 BTU/hr = 200 cu feet/hr
If it is inside, on a 3/4 pipe, if you have 50 feet of pipe, you will lose 2.3 in-H2O pressure from the input to the output. With 1", the pressure drop will be 0.45 in-H2O. Most of the time, you will be better off with the lower pressure drop. See if he can run the pipe as 1" flex line.
Also, make sure the meter can supply the needs of the Hot Water Heater and all your other appliances. Most of the time when we install a NG generator, the meter needs to be upsized.